Ink composition, inkjet recording method, printed material, and process for producing lithographic printing plate

ABSTRACT

An ink composition is provided that includes (A) an N-vinyllactam, (B) a polymerization initiator, and (C) a basic compound, the content of the N-vinyllactam (A) being at least 15 wt % of the total weight of the ink composition. There is also provided an inkjet recording method that includes (a 1 ) a step of discharging the ink composition onto a recording medium, and (b 1 ) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation. Furthermore, a printed material obtained using the ink composition is provided. Moreover, there is provided a process for producing a lithographic printing plate that includes (a 2 ) a step of discharging the ink composition onto a hydrophilic support, and (b 2 ) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation so as to form a hydrophobic image on the hydrophilic support by curing the ink composition.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ink composition suitably used for inkjet recording, an inkjet recording method and, furthermore, a printed material obtained by employing the ink composition and a process for producing a lithographic printing plate.

2. Description of the Related Art

With regard to an image recording method for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic system, sublimation type and melt type thermal transfer systems, an inkjet system, etc. In the electrophotographic system, a process of forming an electrostatic latent image on a photosensitive drum by electrically charging and exposing is required, and the system is complicated; as a result, there is the problem that the production cost is high. With regard to the thermal transfer system, although the equipment is inexpensive, due to the use of an ink ribbon there is the problem that the running cost is high and waste material is generated.

On the other hand, with regard to the inkjet system, the equipment is inexpensive and, since an image is formed directly on a recording medium by discharging an ink only on a required image area, the ink can be used efficiently and the running cost is low. Furthermore, there is little noise and it is excellent as an image recording system.

With regard to an ink composition that can be cured by irradiation with radiation such as ultraviolet rays and, in particular, an inkjet recording ink composition (radiation curing type inkjet recording ink), there is a desire for an ink composition that cures with high sensitivity and forms an image with high image quality. By achieving higher sensitivity, high curability upon exposure to actinic radiation can be imparted, and there are therefore provided various benefits such as a reduction in power consumption, longer lifetime of an actinic radiation generator due to a decrease in the load thereon and, as a result of adequate curing being achieved, suppression of evaporation of uncured low molecular weight material and of a reduction in the strength of an image formed.

Furthermore, there is a desire for an ink composition that gives an image (printed material) that is resistant to cracking, peeling off, etc., and gives a cured film that has excellent flexibility and adhesion to a substrate. A cured film having high flexibility and adhesion to a substrate enables a printed material to be displayed or stored for a long period of time in various environments while maintaining high image quality, and also has advantages such as ease of handling of the printed material.

Furthermore, improvement in the image strength due to higher sensitivity imparts high plate life to an image when the ink composition is used for the formation of an image of a lithographic printing plate.

As an ink composition that cures with high sensitivity and gives a cured film having excellent flexibility and adhesion to a substrate, an ink composition comprising an N-vinyllactam has been disclosed (Japanese Registered Patent No. 2880845). However, the ink composition described in this patent publication is a highly viscous ink composition containing a polymer and an oligomer as main ink components, and it is difficult to discharge by ink jet.

Furthermore, as an ink composition that can be cured by irradiation with radiation such as ultraviolet rays, an ink composition having excellent adhesion has been disclosed (Published Japanese translation of PCT application No. 2004-514014).

Conventionally, when a lithographic printing plate is produced, a so-called PS plate having a constitution in which a lipophilic photosensitive resin layer is provided on a hydrophilic support is used; this photosensitive resin layer is imagewise exposed to light to thus improve or degrade the solubility of the exposed area toward an alkaline developer and form an image, and the non-image area is then dissolved and removed. However, in recent years, a digitization technique of electronically processing, storing and outputting image information using a computer has become widespread, and a new image output method that matches the above technique has been desired. In particular, a method that can produce a printing plate without a treatment employing a developer has been examined, and a process for directly producing a lithographic printing plate using an inkjet recording ink composition has been investigated (ref. e.g. JP-A-54-117203; JP-A denotes a Japanese unexamined patent publication application). In this process, an ink is discharged imagewise on the surface of a preferably hydrophilic support by an inkjet method, etc., and this is then cured by irradiation with actinic radiation, thereby giving a printing plate having a desired image (preferably a hydrophobic image). In order to form an image area of a lithographic printing plate, it is desirable that ink droplets discharged onto a support cure quickly without spreading, and there is currently a desire for an ink composition that is suitable for such an application.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide an ink composition and an inkjet ink composition that ensure long-term storage stability, have excellent curability toward irradiation with actinic radiation, and for which an image obtained by curing the ink has excellent flexibility and adhesion to a substrate, an inkjet recording method employing the ink composition, and a printed material obtained by the inkjet recording method.

It is another object of the present invention to provide a process for producing a lithographic printing plate employing an ink composition that ensures long-term storage stability and has excellent curability toward irradiation with actinic radiation.

The above-mentioned objects have been accomplished by (1), (5), (7), and (8) below. (2) to (4) and (6), which are preferred embodiments, are also shown below.

(1) An ink composition comprising (A) an N-vinyllactam, (B) a polymerization initiator, and (C) a basic compound, the content of the N-vinyllactam (A) being at least 15 wt % of the total weight of the ink composition,

(2) the ink composition according to (1), wherein it comprises (D) a colorant, (E) a dispersant, and (F) a surfactant,

(3) the ink composition according to (1) or (2), wherein the dispersant is a polymeric dispersant,

(4) the ink composition according to any one of (1) to (3), wherein the polymeric dispersant has an amine value that is larger than its acid value,

(5) the ink composition according to (4), wherein the difference between the acid value and the amine value of the polymeric dispersant is at least 5 mg KOH/g,

(6) the ink composition according to any one of (1) to (5), wherein the N-vinyllactam is N-vinylcaprolactam,

(7) the ink composition according to any one of (1) to (6), wherein the content of N-vinyllactam is no greater than 40 wt % of the total weight of the ink composition,

(8) the ink composition according to any one of (1) to (7), wherein the basic compound is an organic amine compound,

(9) the ink composition according to any one of (1) to (8), wherein the basic compound is at least one selected from the group consisting of an organic amine compound containing a non-acid polar group, an organic amine compound having a polymerizable group, and a hindered amine-based compound, (10) the ink composition according to any one of (1) to (9), wherein the viscosity of the ink composition at 25° C. is 3 to 50 mPa·s, (11) the ink composition according to any one of (1) to (10), wherein the viscosity of the ink composition at 25° C. is 7 to 30 mPa·s, (12) the ink composition according to any one of (1) to (11), wherein it is for inkjet recording, (13) an inkjet recording method comprising (a¹) a step of discharging the ink composition according to any one of (1) to (12) onto a recording medium, and (b¹) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation, (14) the inkjet recording method according to (13), wherein the actinic radiation is UV radiation having a peak light emission wavelength in the range of 350 to 420 nm and is emitted by a UV radiation-emitting light-emitting diode that gives a maximum illumination intensity on the surface of a recording medium of 10 to 2,000 mW/cm², (15) a printed material recorded by the inkjet recording method according to (13) or (14), (16) a process for producing a lithographic printing plate, the process comprising (a²) a step of discharging the ink composition according to any one of (1) to (12) onto a hydrophilic support, and (b²) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation so as to form a hydrophobic image on the hydrophilic support by curing the ink composition.

DETAILED DESCRIPTION OF THE INVENTION (1) Ink Composition

The ink composition of the present invention (hereinafter, also called simply an ‘ink’) comprises (A) an N-vinyllactam, (B) a polymerization initiator, and (C) a basic compound, the content of the N-vinyllactam (A) being at least 15 wt % of the total weight of the ink composition.

Furthermore, the ink composition of the present invention preferably comprises (D) a colorant, (E) a dispersant, and (F) a surfactant.

The present invention is explained in detail below.

The ‘radiation’ referred to in the present invention is not particularly limited as long as it is actinic radiation that can provide energy allowing an initiator species to be generated in an ink composition by the radiation, and broadly includes α rays, γ rays, X rays, ultraviolet rays, visible rays, and electron beams and, among these, ultraviolet rays and electron beams are preferable from the viewpoint of curing sensitivity and the availability of equipment, and ultraviolet rays are particularly preferable. With regard to the ink composition of the present invention, an ink composition that can be cured when exposed to ultraviolet rays as radiation is therefore preferable.

(A) N-vinyllactam

The ink composition of the present invention comprises an N-vinyllactam. Preferred examples of the N-vinyllactam include compounds represented by Formula (I) below. In some of the compound examples below, the hydrocarbon chain is described by a simplified structural formula in which symbols for carbon (C) and hydrogen (H) are omitted.

In Formula (I), n denotes an integer of 1 to 5; n is preferably an integer of 2 to 4 from the viewpoint of flexibility after the ink composition is cured, adhesion to a recording medium, and ease of availability of starting material, n is more preferably an integer of 2 or 4, and n is particularly preferably 4, which is N-vinylcaprolactam. N-vinylcaprolactam is preferable since it has excellent safety, is commonly used and easily available at a relatively low price, and gives particularly good ink curability and adhesion of a cured film to a recording medium.

The N-vinyllactam may have a substituent such as an alkyl group or an aryl group on the lactam ring, and may have a saturated or unsaturated ring structure bonded thereto.

The ink composition of the present invention comprises an N-vinyllactam at 15 wt % or greater of the total weight of the ink composition. When the content of the N-vinyllactam is less than 15 wt %, it is not possible to obtain an ink composition that has sufficient curability and gives a cured film having flexibility and excellent adhesion to a substrate. It is preferable for the content of the N-vinyllactam in the ink composition to be no greater than 40 wt %, more preferably at least 15 wt % but no greater than 35 wt %, and yet more preferably at least 18 wt % but no greater than 30 wt %.

The N-vinyllactam is a compound having a relatively high melting point. It is preferable for the content to be no greater than 40 wt % since good solubility is exhibited even at a low temperature of 0° C. or less and the temperature range in which the ink composition can be handled becomes large.

The N-vinyllactam may be contained in the ink composition singly or in a combination of a plurality of types thereof. When it contains a plurality of types of N-vinyllactam, the ink composition of the present invention comprises, as a total amount, the N-vinyllactams at 15 wt % or greater of the total weight of the ink composition, and the content of each N-vinyllactam may be less than 15 wt %.

Other Polymerizable Compound

It is preferable for the ink composition of the present invention to employ in combination another polymerizable compound in addition to the N-vinyllactam. Examples of the polymerizable compound that can be used in combination include a radically polymerizable compound and a cationically polymerizable compound, and among them it is preferable to use a radically polymerizable compound in combination.

The combined use of a radically polymerizable compound enables an ink composition having better curability to be provided, which is preferable.

The ‘radically polymerizable compound’ referred to in the present invention naturally means a radically polymerizable compound other than an N-vinyllactam.

Radically Polymerizable Compound

As the radically polymerizable compound, a photocuring material is known that employs a photopolymerizable composition described in, for example, JP-A-7-159983, JP-B-7-31399, JP-A-8-224982, JP-A-10-863, etc. (JP-B denotes a Japanese examined patent application publication).

The radically polymerizable compound is a compound having a radically polymerizable ethylenically unsaturated bond, and may be any compound as long as it has at least one radically polymerizable ethylenically unsaturated bond in the molecule; examples thereof include those having a chemical configuration such as a monomer, an oligomer, or a polymer. One type of radically polymerizable compound may be used, or two or more types thereof may be used in combination in order to improve an intended property.

Examples of the polymerizable compound having a radically polymerizable ethylenically unsaturated bond include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, crotonoic acid, isocrotonoic acid, and maleic acid, and esters and salts thereof, anhydrides having an ethylenically unsaturated group, acrylonitrile, styrene, and various types of radically polymerizable compounds such as unsaturated polyesters, unsaturated polyethers, unsaturated polyamides, unsaturated urethanes, (meth)acryl type monomer or prepolymer and (meth)acrylate esters of epoxy type monomer or prepolymer, urethane type monomer or prepolymer.

Herein, (meth)acrylate is an abbreviated description of methacrylate and acrylate.

Specific examples thereof include (poly)ethylene glycol mono(meth)acrylate, (poly)ethylene glycol(meth)acrylate methyl ester, (poly)ethylene glycol(meth)acrylate ethyl ester, (poly)ethylene glycol(meth)acrylate phenyl ester, (poly)propylene glycol mono(meth)acrylate, (poly)propylene glycol mono(meth)acrylate phenyl ester, (poly)propylene glycol(meth)acrylate methyl ester, (poly)propylene glycol(meth)acrylate ethyl ester, (poly)propylene glycol diglycidyl ether acrylic acid adduct, neopentyl glycol diacrylate, (poly)ethylene glycol di(meth)acrylate, (poly)tetramethylene glycol di(meth)acrylate, bisphenol A PO (propylene oxide) adduct di(meth)acrylate, ethoxylated neopentyl glycol diacrylate, propoxylated neopentyl glycol diacrylate, bisphenol A EO (ethylene oxide) adduct di(meth)acrylate, EO-modified pentaerythritol tri(meth)acrylate, PO-modified pentaerythritol tri(meth)acrylate, EO-modified pentaerythritol tetra(meth)acrylate, PO-modified pentaerythritol tetra(meth)acrylate, EO-modified dipentaerythritol tetra(meth)acrylate, PO-modified dipentaerythritol tetra(meth)acrylate, EO-modified trimethylolpropane tri(meth)acrylate, PO-modified trimethylolpropane tri(meth)acrylate, EO-modified tetramethylolmethane tetra(meth)acrylate, and PO-modified tetramethylolmethane tetra(meth)acrylate, acrylic acid derivatives such as 2-ethylhexyl acrylate, n-octyl acrylate, n-nonyl acrylate, n-decyl acrylate, isooctyl acrylate, n-lauryl acrylate, n-tridecyl acrylate, cetyl acrylate, n-stearyl acrylate, isomyristyl acrylate, isostearyl acrylate, 2-hydroxyethyl acrylate, butoxyethyl acrylate, tetrahydrofurfuryl acrylate, benzyl acrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol tetraacrylate, trimethylolpropane triacrylate, tetramethylolmethane tetraacrylate, oligoester acrylate, isoamyl acrylate, N-methylol acrylamide, diacetone acrylamide, epoxy acrylate, methacrylic acid derivatives such as methylmethacrylate, n-butyl methacrylate, 2-ethylhexyl methacrylate, n-octyl methacrylate, n-nonyl methacrylate, n-decyl methacrylate, isooctyl methacrylate, n-lauryl methacrylate, n-tridecyl methacrylate, n-cetyl methacrylate, n-stearyl methacrylate, allylmethacrylate, glycidyl methacrylate, benzyl methacrylate, dimethylaminomethyl methacrylate, trimethylolethane trimethacrylate, trimethylolpropane trimethacrylate and 2,2-bis(4-methacryloxypolyethoxyphenyl)propane, and allyl compound derivatives such as allyl glycidyl eter, diallyl phthalate and triallyltrimellitate. Other specific examples thereof include 1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, 1,10-decanediol diacrylate, 2-ethylhexyl-diglycol acrylate, 2-hydroxy-3-phenoxypropyl acrylate, 2-hydroxybutyl acrylate, neopentylglycol diacrylate hydroxypivalate, 2-acryloyloxyethylphthalic acid, EO-modified nonylphenol acrylate, methoxy-polyethyleneglycol acrylate, tetramethylolmethane triacrylate, 2-acryloyloxyethyl-2-hydroxyethylphthalic acid, dimethyloltricyclodecane diacrylate, ethoxylated phenylacrylate, 2-acryloyloxyethylsuccinic acid, modified glycerol triacrylate, bisphenol A diglycigyl ether acrylic acid adduct, modified bisphenol A diacrylate, phenoxy-polyethylene glycol acrylate, 2-acryloyloxyethylhexahydrophthalic acid, dipentaerythritol hexaacrylate, pentaerythritol triacrylate tolylenediisocyanate urethane prepolymer, lactone modified flexible acrylate, butoxyethyl acrylate, pentaerythritol triacrylate hexametylenediisocyanate urethane prepolymer, 2-hydroxyethyl acrylate, methoxydipropylene grycol acrylate, ditrimetylolpropane tetraacrylate, and pentaerythritol triacrylate hexametylenediisocyanate urethane prepolymer, etc.

More specifically, commercial products, radically polymerizable or crosslinking monomers, oligomers, and polymers known in the art such as those described in ‘Kakyozai Handobukku’ (Crosslinking Agent Handbook), Ed. S. Yamashita (Taiseisha, 1981); ‘UV•EB Koka Handobukku’ (UV•EB Curing Handbook (Starting Materials) Ed. K. Kato (Kobunshi Kankoukai, 1985); ‘UV•EB Koka Gijutsu no Oyo to Shijyo’ (Application and Market of UV•EB Curing Technology’, p. 79, Ed. Rad Tech (CMC, 1989); and E. Takiyama ‘Poriesuteru Jushi Handobukku’ (Polyester Resin Handbook), (The Nikkan Kogyo Shimbun Ltd., 1988) can be used.

These acrylate compounds can be reduced viscosity, can be obtained stable ink dischargability, and have high polymerizable sensitivity and good adhesion to a recording medium than a polymerizable compound having been used for conventional UV curing type ink, and that is preferable.

It is also preferable to use a vinyl ether compound as the radically polymerizable compound. Examples of vinyl ether compounds that are suitably used include di- or tri-vinyl ether compounds such as ethylene glycol divinyl ether, ethylene glycol monovinyl ether, diethylene glycol divinyl ether, triethylene glycol monovinyl ether, triethylene glycol divinyl ether, propylene glycol divinyl ether, dipropylene glycol divinyl ether, butanediol divinyl ether, hexanediol divinyl ether, cyclohexanedimethanol divinyl ether, hydroxyethyl monovinyl ether, hydroxynonyl monovinyl ether, and trimethylolpropane trivinyl ether; and monovinyl ether compounds such as ethyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, octadecyl vinyl ether, cyclohexyl vinyl ether, hydroxybutyl vinyl ether, 2-ethylhexyl vinyl ether, cyclohexanedimethanol monovinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, isopropenyl vinyl ether, dodecyl vinyl ether, diethylene glycol monovinyl ether, and octadecyl vinyl ether.

Among these vinyl ether compounds, from the viewpoint of curability, adhesion, and surface hardness, a divinyl ether compound and a trivinyl ether compound are preferable, and a divinyl ether compound is particularly preferable. The vinyl ether compounds may be used singly or in a combination of two or more types as appropriate.

Furthermore, in the present invention, it is also preferable to use a radically polymerizable compound having a polycyclic structure as the radically polymerizable compound used in combination with the N-vinyllactam.

Specifically, a radically polymerizable compound having a dicyclopentanyl structure, a dicyclopentenyl structure, or an adamantyl structure is preferable. It is preferable to use a radically polymerizable compound having a polycyclic structure since it has high reactivity, low viscosity, and excellent adhesion to a recording medium.

Examples of the radically polymerizable compound having a polycyclic structure that can be used preferably in the present invention are listed below, but the present invention is not limited thereby.

The content ratio by weight of the N-vinyllactam (A) to the radically polymerizable compound that can be used in combination in the ink is preferably N-vinyllactam: radically polymerizable compound that can be used in combination=1:6.3 to 1:1, more preferably 1:6 to 1:1, and particularly preferably 1:5 to 1:1. It is preferable to combine them in the above-mentioned range since good curability, flexibility of a cured film, and adhesion of the cured film to a substrate are obtained.

In the present invention, the radically polymerizable compound may be used in combination with an oligomer or a polymer. The oligomer referred to here means a compound having a molecular weight (a weight-average molecular weight for one having a molecular weight distribution) of 2,000 or greater, and the polymer referred to here means a compound having a molecular weight (a weight-average molecular weight for one having a molecular weight distribution) of 10,000 or greater. The oligomer and the polymer optionally have a radically polymerizable group. It is preferable for the oligomer and the polymer to have no more than 4 radically polymerizable groups per molecule (an average of no more than 4 over all the molecules contained for one having a molecular weight distribution) since an ink composition having excellent flexibility can be obtained. They can suitably be used from the viewpoint of adjusting the viscosity to a level most suitable for jetting the ink.

Cationically Polymerizable Compound

The ink composition of the present invention may comprise in combination as necessary a cationic polymerizable compound as another polymerizable compound. When a cationic polymerizable compound is used in combination, it is preferable to use a cationic polymerization initiator in combination as a polymerization initiator.

The cationically polymerizable compound used in the present invention is not particularly limited as long as it is a compound that undergoes a polymerization reaction by virtue of an acid generated by the photo-acid generator and is cured, and various types of cationically polymerizable monomers known as photo-cationically polymerizable monomers may be used. Examples of the cationically polymerizable monomer include epoxy compounds, vinyl ether compounds, oxetane compounds described in JP-A-6-9714, JP-A-2001-31892, JP-A-2001-40068, JP-A-2001-55507, JP-A-2001-310938, JP-A-2001-310937, JP-A-2001-220526, etc.

As the cationically polymerizable compound, for example, a cationically polymerizable type photocuring resin is known, and in recent years cationically photopolymerizable type photocuring resins sensitized to a visible light wavelength region of 400 nm or longer have been disclosed in, for example, JP-A-6-43633 and JP-A-8-324137. They may also be applied to the ink composition of the present invention.

(B) Polymerization Initiator

In the present invention, when the ink is cured using actinic radiation such as ultraviolet rays, it comprises a polymerization initiator. As a polymerization initiator that can be used in the present invention, a known polymerization initiator may be used, and it is preferable to use a radical polymerization initiator. The polymerization initiator that can be used in the present invention may be used singly or in a combination of two or more types. Furthermore, the radical polymerization initiator may be used in combination with a cationic polymerization initiator.

The polymerization initiator that can be used in the ink composition of the present invention is a compound that forms a polymerization initiating species by absorbing external energy. The external energy used for initiating polymerization can be broadly divided into heat and actinic radiation, and a thermal polymerization initiator and a photopolymerization initiator are used respectively. Examples of the actinic radiation include γ rays, β rays, an electron beam, ultraviolet rays, visible light, and infrared rays.

Radical Polymerization Initiator

Examples of the radical polymerization initiator that can be used in the present invention include (a) an aromatic ketone, (b) an acylphosphine compound, (c) an aromatic onium salt compound, (d) an organic peroxide, (e) a thio compound, (f) a hexaarylbiimidazole compound, (g) a ketoxime ester compound, (h) a borate compound, (i) an azinium compound, (j) a metallocene compound, (k) an active ester compound, (l) a compound having a carbon-halogen bond, and (m) an alkylamine compound. With regard to these radical polymerization initiators, the above-mentioned compounds (a) to (m) may be used singly or in combination.

The radical polymerization initiator in the present invention may suitably be used singly or in a combination of two or more types.

Preferred examples of the aromatic ketone (a), the acylphosphine compound (b) and the thio compoud (e) include a compound having a benzophenone skeleton (benzophenone compound) or a compound having a thioxanthone skeleton (thioxanthone compound) described in ‘RADIATION CURING IN POLYMER SCIENCE AND TECHNOLOGY’ J. P. FOUASSIER and J. F. RABEK (1993), pp. 77 to 117. Preferred examples include an α-thiobenzophenone compound described in JP-B-47-6416, a benzoin ether compound described in JP-B-47-3981, an α-substituted benzoin compound described in JP-B-47-22326, a benzoin derivative described in JP-B-47-23664, an aroylphosphonic acid ester described in JP-A-57-30704, a dialkoxybenzophenone described in JP-B-60-26483, benzoin ethers described in JP-B-60-26403 and JP-A-62-81345, α-aminobenzophenones described in JP-B-1-34242, U.S. Pat. No. 4,318,791, and EP No. 0284561A1, p-di(dimethylaminobenzoyl)benzene described in JP-A-2-211452, a thio-substituted aromatic ketone described in JP-A-61-194062, an acylphosphine sulfide described in JP-B-2-9597, an acylphosphine described in JP-B-2-9596, a thioxanthone described in JP-B-63-61950, and a coumarin described in JP-B-59-42864.

As the aromatic onium salt compound (c), there can be cited aromatic onium salts of elements of Groups 15, 16, and 17 of the periodic table, specifically, N, P, As, Sb, Bi, O, S, Se, Te, and I. Examples thereof include iodonium salts described in EP No. 104143, U.S. Pat. No. 4,837,124, JP-A-2-150848, and JP-A-2-96514, diazonium salts (optionally substituted benzenediazoniums, etc.) described in EP Nos. 370693, 233567, 297443, 297442, 279210, and 422570, U.S. Pat. Nos. 3,902,144, 4,933,377, 4,760,013, 4,734,444, and 2,833,827, diazonium salt resins (diazodiphenylamine formaldehyde resins, etc.), N-alkoxypyridinium salts, etc. (e.g. those described in U.S. Pat. No. 4,743,528, JP-A-63-138345, JP-A-63-142345, JP-A-63-142346, and JP-B-46-42363; specific examples thereof include 1-methoxy-4-phenylpyridinium tetrafluoroborate); furthermore, compounds described in JP-B-52-147277, 52-14278, and 52-14279 may suitably be used. A radical or an acid is formed as an active species.

As the organic peroxide (d), almost all organic compounds having at least one oxygen-oxygen bond per molecule can be cited, and preferred examples thereof include peroxide ester compounds such as 3,3′,4,4′-tetra(t-butylperoxycarbonyl)benzophenone, 3,3′,4,4′-tetra(t-amylperoxycarbonyl)benzophenone, 3,3′,4,4′-tetra(t-hexylperoxycarbonyl)benzophenone, 3,3′,4,4′-tetra(t-octylperoxycarbonyl)benzophenone, 3,3′,4,4′-tetra(cumylperoxycarbonyl)benzophenone, 3,3′,4,4′-tetra(p-isopropylcumylperoxycarbonyl)benzophenone, and di-t-butyldiperoxyisophthalate.

As the hexaarylbiimidazole compound (f), there can be cited lophine dimers described in JP-B-45-37377 and JP-B-44-86516, and examples thereof include 2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetraphenylbiimidazole, 2,2′-bis(o-bromophenyl)-4,4′,5,5′-tetraphenylbiimidazole, 2,2′-bis(o,p-dichlorophenyl)-4,4′,5,5′-tetraphenylbiimidazole, 2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetra(m-methoxyphenyl)biimidazole, 2,2′-bis(o,o′-dichlorophenyl)-4,4′,5,5′-tetraphenylbiimidazole, 2,2′-bis(o-nitrophenyl)-4,4′,5,5′-tetraphenylbiimidazole, 2,2′-bis(o-methylphenyl)-4,4′,5,5′-tetraphenylbiimidazole, and 2,2′-bis(o-trifluorophenyl)-4,4′,5,5′-tetraphenylbiimidazole.

As the ketoxime ester compound (g), there can be cited 3-benzoyloxyiminobutan-2-one, 3-acetoxyiminobutan-2-one, 3-propionyloxyiminobutan-2-one, 2-acetoxyiminopentan-3-one, 2-acetoxyimino-1-phenylpropan-1-one, 2-benzoyloxyimino-1-phenylpropan-1-one, 3-p-toluenesulfonyloxyiminobutan-2-one, and 2-ethoxycarbonyloxyimino-1-phenylpropan-1-one.

Examples of the borate compound (h) include compounds described in U.S. Pat. Nos. 3,567,453 and 4,343,891, and EP Nos. 109,772 and 109,773.

Examples of the azinium salt compound (i) include N—O bond-containing compounds described in JP-A-63-138345, JP-A-63-142345, JP-A-63-142346, JP-A-63-143537, and JP-B-46-42363.

Examples of the metallocene compound (j) include titanocene compounds described in JP-A-59-152396, JP-A-61-151197, JP-A-63-41484, JP-A-2-249, and JP-A-2-4705, and iron-arene complexes described in JP-A-1-304453 and JP-A-1-152109.

Specific examples of the titanocene compound include dichlorobis(cyclopentadienyl)titanium, bis(cyclopentadienyl)bis(phenyl)titanium, bis(cyclopentadienyl)bis(2,3,4,5,6-pentafluorophen-1-yl)titanium, bis(cyclopentadienyl)bis(2,3,5,6-tetrafluorophen-1-yl)titanium, bis(cyclopentadienyl)bis(2,4,6-trifluorophen-1-yl)titanium, bis(cyclopentadienyl)bis(2,6-difluorophen-1-yl)titanium, bis(cyclopentadienyl)bis(2,4-difluorophen-1-yl)titanium, bis(methylcyclopentadienyl)bis(2,3,4,5,6-pentafluorophen-1-yl)titanium, bis(methylcyclopentadienyl)bis(2,3,5,6-tetrafluorophen-1-yl)titanium, bis(methylcyclopentadienyl)bis(2,4-difluorophen-1-yl)titanium, bis(cyclopentadienyl)bis[2,6-difluoro-3-(pyrr-1-yl)phenyl]titanium, bis(cyclopentadienyl)bis[2,6-difluoro-3-(methylsulfonamido)phenyl]titanium, and bis(cyclopentadienyl)bis[2,6-difluoro-3-(N-butylbiaroylamino)phenyl]titanium.

Examples of the active ester compound (k) include nitrobenzyl ester compounds described in EP Nos. 0290750, 046083, 156153, 271851, and 0388343, U.S. Pat. Nos. 3,901,710 and 4,181,531, JP-A-60-198538, and JP-A-53-133022, iminosulfonate compounds described in EP Nos. 0199672, 84515, 199672, 044115, and 0101122, U.S. Pat. Nos. 4,618,564, 4,371,605, and 4,431,774, JP-A-64-18143, JP-A-2-245756, and JP-A-4-365048, and compounds described in JP-B-62-6223, JP-B-63-14340, and JP-A-59-174831.

Preferred examples of the compound (l) having a carbon-halogen bond include a compound described in Wakabayashi et. al, Bull. Chem. Soc. Japan, 42, 2924 (1969), a compound described in British Patent No. 1388492, a compound described in JP-A-53-133428, and a compound described in German Patent No. 3337024.

Examples further include a compound described in F. C. Schaefer et al., J. Org. Chem., 29, 1527 (1964), a compound described in JP-A-62-58241, a compound described in JP-A-5-281728, a compound described in German Pat. No. 2641100, a compound described in German Pat. No. 3333450, compounds described in German Pat. No. 3021590, and compounds described in German Pat. No. 3021599.

Cationic Polymerization Initiator

In the ink composition of the present invention, when a cationically polymerizable compound is used in combination, it is preferable to use a cationic polymerization initiator in combination.

Firstly, B(C₆F₅)₄ ⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, and CF₃SO₃ ⁻ salts of diazonium, ammonium, iodonium, sulfonium, phosphonium, etc. aromatic onium compounds can be cited. Secondly, sulfonated materials that generate a sulfonic acid can be cited. Thirdly, halides that photogenerate a hydrogen halide can also be used. Fourthly, iron arene complexes can be cited.

Examples [(b-1) to (b-96)] of cationic polymerization initiators that are suitably used in the present invention are listed below, but the present invention should not be construed as being limited thereby.

In the ink composition of the present invention, the total amount of polymerization initiator used is preferably 0.01 to 35 wt % relative to the total amount of polymerizable compound, including an N-vinyllactam, used, more preferably 0.5 to 20 wt %, and yet more preferably 1.0 to 15 wt %. The ink composition can be cured with 0.01 wt % or greater of the polymerization initiator, and a cured film having a uniform degree of curing can be obtained with 35 wt % or less.

Furthermore, when a sensitizing colorant, which will be described later, is used in the ink composition of the present invention, the total amount of polymerization initiator used is preferably 200:1 to 1:200 relative to the sensitizing colorant as a ratio by weight of polymerization initiator: sensitizing colorant, more preferably 50:1 to 1:50, and yet more preferably 20:1 to 1:5.

(C) Basic Compound

The ink composition of the present invention comprises a basic compound. By containing the basic compound, an ink composition having excellent long-term storage stability can be obtained.

The basic compound used in the present invention is not particularly limited, and it is possible to use an inorganic basic compound or an organic basic compound, but it is preferable to use an organic basic compound.

It is preferable for the organic basic compound to have a higher basicity than phenol. The organic basic compound is preferably a nitrogen-containing organic basic compound, and more preferably an organic amine compound. In particular, it is preferable to use an organic amine compound containing a non-acid polar group such as a hydroxyl group, a cyano group, an ether group, or an amide group. It is also preferable to use an organic amine compound having at least two amino groups per molecule. It is more preferable to use an organic amine compound having a polymerizable group. The use of an organic amine compound having a polymerizable group is particularly preferable since, when the ink composition is cured by irradiation with actinic radiation, the organic amine compound also cures and does not remain in the film.

Preferred specific examples of the basic compound in the present invention include decylamine, dodecylamine, N,N-dimethyldodecylamine, stearylamine, cetylamine, benzylpiperidine, N,N-dimethylcyclohexylamine, mono-, di-, or tri-ethanolamine, aminopropanol, aminobutanol, aminohexanol, dimethylaminohexanol, morpholine, aminoethylmorpholine, aminopropylmorpholine, aminoethylpiperazine, aminoethylpyrrolidine, bis(hydroxyethyl)piperazine, aminopropylpyrrolidinone, aminoethoxyethanol, dimethylaminoethylmorpholine, phenylmorpholine, 1,3-bis[1-(2-hydroxyethyl)-4-piperidyl]propane, gramine, 1-(2-phenethyl)-4-piperidone, ethylenediamine, trimethylenediamine, tetramethylenediamine, hexamethylenediamine, aniline, N,N-diethylaniline, dodecylaniline, aminobiphenyl, aminophenol, 4-aminoacetanilide, aminoacetophenone, aminobenzamide, aminobenzanilide, aminobenzophenone, aminopyridine, dimethylaminomethylthiophene, dimethylaminophenethyl alcohol, polyethyleneimine, polyarylamine, polyvinylpyridine, a copolymer of N,N-dimethylaminoethyl methacrylate and a methacrylic acid ester (e.g. butyl methacrylate), a copolymer of N,N-diethylaminoethyl methacrylate and an acrylic acid ester (e.g. ethyl acrylate), a copolymer of dimethylaminomethylstyrene and styrene, and a condensed polymer of 4-hydroxy-2,2,6,6-tetramethyl-1-piperidineethanol and 1,4-butanedioic acid.

It is also possible to use, as the amine having a polymerizable group, N,N-dimethylaminoethyl(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, and a so-called aminoacrylate in which a polyfunctional acrylate monomer or polyester acrylate is modified with an amine.

Amines having a polymerizable group are commercially available, and examples thereof include EBECRYL P115 and EBECRYL 7100 (Daicel-UCB Co., Ltd.), dimethylaminoethyl acrylate (DMA, Osaka Organic Chemical Industry Ltd.), dimethylaminoethyl methacrylate (Light-ester DM, Kyoeisha Chemical Co., Ltd.), and diethylaminoethyl methacrylate (Light-ester DE, Kyoeisha Chemical Co., Ltd.).

As nitrogen-containing basic compounds that can preferably be used as the basic compound, for example, the structures represented by (A) to (E) below can be cited.

Here, R²⁵⁰, R²⁵¹, and R²⁵² independently denote a hydrogen atom, an alkyl group having 1 to 6 carbons, an aminoalkyl group having 1 to 6 carbons, a hydroxyalkyl group having 1 to 6 carbons, or a substituted or unsubstituted aryl group having 6 to 20 carbons, and R²⁵¹ and R²⁵² may be bonded to each other to form a ring.

In the formulae, R²⁵³, R²⁵⁴, R²⁵⁵, and R²⁵⁶ independently denote an alkyl group having 1 to 6 carbons.

A more preferred compound is a nitrogen-containing basic compound having in one molecule at least two nitrogen atoms that are in different chemical environments, and a particularly preferred compound is a compound having both a substituted or unsubstituted amino group and a nitrogen-containing cyclic structure or a compound having an alkylamino group. Preferred specific examples thereof include a substituted or unsubstituted guanidine, a substituted or unsubstituted aminopyridine, a substituted or unsubstituted aminoalkylpyridine, a substituted or unsubstituted aminopyrrolidine, a substituted or unsubstituted indazole, a substituted or unsubstituted pyrazole, a substituted or unsubstituted pyrazine, a substituted or unsubstituted pyrimidine, a substituted or unsubstituted purine, a substituted or unsubstituted imidazoline, a substituted or unsubstituted pyrazoline, a substituted or unsubstituted piperazine, a substituted or unsubstituted aminomorpholine, and a substituted or unsubstituted aminoalkylmorpholine. Preferred substituents are an amino group, an aminoalkyl group, an alkylamino group, an aminoaryl group, an arylamino group, an alkyl group, an alkoxy group, an acyl group, an acyloxy group, an aryl group, an aryloxy group, a nitro group, a hydroxyl group, and a cyano group.

Preferred specific examples of the nitrogen-containing basic compound include guanidine, 1,1-dimethylguanidine, 1,1,3,3-tetramethylguanidine, 2-aminopyridine, 3-aminopyridine, 4-aminopyridine, 2-dimethylaminopyridine, 4-dimethylaminopyridine, 2-diethylaminopyridine, 2-(aminomethyl)pyridine, 2-amino-3-methylpyridine, 2-amino-4-methylpyridine, 2-amino-5-methylpyridine, 2-amino-6-methylpyridine, 3-aminoethylpyridine, 4-aminoethylpyridine, 3-aminopyrrolidine, piperazine, N-(2-aminoethyl)piperazine, N-(2-aminoethyl)piperidine, 4-amino-2,2,6,6-tetramethylpiperidine, 4-piperidinopiperidine, 2-iminopiperidine, 1-(2-aminoethyl)pyrrolidine, pyrazole, 3-amino-5-methylpyrazole, 5-amino-3-methyl-1-p-tolylpyrazole, pyrazine, 2-(aminomethyl)-5-methylpyrazine, pyrimidine, 2,4-diaminopyrimidine, 4,6-dihydroxypyrimidine, 2-pyrazoline, 3-pyrazoline, N-aminomorpholine, N-(2-aminoethyl)morpholine, 1,5-diazabicyclo[4.3.0]non-5-ene, 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,4-diazabicyclo[2.2.2]octane, 2,4,5-triphenylimidazole, N-methylmorpholine, N-ethylmorpholine, N-hydroxyethylmorpholine, N-benzylmorpholine, and a tertiary morpholine derivative such as cyclohexylmorpholinoethylthiourea (CHMETU), but the examples are not limited thereto.

Furthermore, as an example of a nitrogen-containing basic compound that can be suitably used in the present invention, a hindered amine-based compound can be cited. The hindered amine-based compound can suitably be used since it also functions as a polymerization inhibitor.

The hindered amine-based compound is a compound having a moiety having a hindered amine structure in the molecule; examples thereof include those described in JP-A-61-91257, JP-A-11-52575 (e.g. those described in [0005] of the publication), and JP-A-2003-246138, and a representative compound thereof is a 2,2,6,6-tetramethylpiperidine derivative having a structure in which all the hydrogen atoms on the 2- and 6-carbons of the piperidine are replaced by methyl groups.

In the present invention, a hindered amine-based compound represented by Formula (II) below may preferably be used.

In Formula (II), Y denotes a non-metallic atomic group necessary for forming a 5- to 7-membered ring together with C and N. X denotes a hydrogen atom, an aliphatic group, an aromatic group, an acyl group, an alkylsulfonyl group, an arylsulfonyl group, an alkylsulfinyl group, an arylsulfinyl group, a hydroxyl group, an aliphatic group-substituted oxy group, an aromatic group-substituted oxy group, an acyloxy group, an alkylsulfonyloxy group, an arylsulfonyloxy group, or an oxy radical group. R⁷ to R¹⁰ may be identical to or different from each other and each denotes a hydrogen atom or an alkyl group. Here, any two groups of R⁷ to R¹⁰ and Y may be bonded to each other to form a 5- to 7-membered ring.

In Formula (II), preferred examples of the 5- to 7-membered ring formed by Y include a pyrrolidine ring, a piperazine ring, a morpholine ring, and a piperidine ring.

In Formula (II), examples of the aliphatic group denoted by X include an alkyl group, an alkenyl group, an alkynyl group, and an aralkyl group, and they may further have a substituent. The aliphatic group may be an open-chain aliphatic group or a cyclic aliphatic group, and the open-chain aliphatic group may further be branched. Among them, an alkyl group and a substituted alkyl group are particularly preferable. Examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an iso-propyl group, an n-butyl group, a t-butyl group, an n-octyl group, a benzyl group, and a hexadecyl group. Examples of the alkenyl group include an allyl group and an oleyl group, and examples of the alkynyl group include an ethynyl group. Examples of the aromatic group include an aryl group and a substituted aryl group, the aryl group preferably having 6 to 30 carbons, and more preferably 6 to 20 carbons. With regard to the number of carbons in the aryl moiety of the substituted aryl group, the above-mentioned range is also preferable. Specific examples of the aryl group include a phenyl group, an α-naphthyl group, and a β-naphthyl group. Examples of the acyl group include an acetyl group, a benzoyl group, and a pentanoyl group. The alkylsulfonyl group is preferably an alkylsulfonyl group having no greater than 30 carbons, and examples thereof include a methylsulfonyl group, a trifluoromethylsulfonyl group, an ethylsulfonyl group, a butylsulfonyl group, and a dodecylsulfonyl group. Examples of the arylsulfonyl group include a benzenesulfonyl group, a toluenesulfonyl group, and a naphthalenesulfonyl group. Examples of the alkylsulfinyl group include a methanesulfinyl group, and examples of the arylsulfinyl group include a benzenesulfinyl group. Examples of the aliphatic group-substituted oxy group include oxy groups substituted with an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, etc. Examples of the aromatic group-substituted oxy group include oxy groups substituted with an aryl group, a substituted aryl group, etc. Examples of the acyloxy group include an acetyloxy group and a benzoyloxy group.

R⁷ to R¹⁰ in Formula (II) denote a hydrogen atom or an alkyl group (this alkyl group is selected from the same range as for the alkyl group denoted by X above).

The compound represented by Formula (II) above may be synthesized easily in accordance with methods described in Journal of Synthetic Organic Chemistry, Japan, 29 (4), 366 (1971), JP-A-49-53571, JP-A-49-53572, JP-A-49-53573, JP-A-49-53574, JP-B-49-20974, EP-A-264,730, U.S. Pat. No. 4,639,415, etc.

Furthermore, in the present invention, a hindered amine-based compound represented by Formula (III) below in particular may be preferably used.

In Formula (III), R³¹ denotes a hydrogen atom, an aliphatic group, —OR³² (said R³² denotes a hydrogen atom, an aliphatic group, or an acyl group), —O., or an acyl group. Z¹ denotes a hydrogen atom, —OR³³ (said R³³ denotes a hydrogen atom, an aliphatic group, an aromatic group, an alkoxycarbonyl group, an acyl group, or an aminocarbonyl group), —NR³⁴R³⁵ (said R³⁴ and R³⁵ independently denote a hydrogen atom, an aliphatic group, an aromatic group, an acyl group, an aminocarbonyl group, or a sulfonyl group), —COOR³⁶ (said R³⁶ denotes a hydrogen atom, an aliphatic group, or an aromatic group), a halogen atom, an aliphatic group, or an aromatic group. Z² denotes a hydrogen atom, an aliphatic group, an aromatic group, or —OR³⁷ (said R³⁷ denotes a hydrogen atom or an aliphatic group). Z¹ and Z² may form a carbonyl group in a form containing the carbon atom substituted by Z¹ and Z², or may be bonded to each other to form a cyclic structure. At least one of Z¹ and Z² may denote a group that may form a bond as a result of a reaction with a reactive group contained in a polymer, an oligomer, or a low molecular weight compound.

When R³¹ in Formula (III) denotes an aliphatic group, examples of this aliphatic group include an alkyl group, an alkenyl group, an alkynyl group, and an aralkyl group, and they may further have a substituent. Among them, an alkyl group, a substituted alkyl group, an alkenyl group, a substituted alkenyl group, an aralkyl group, and a substituted aralkyl group are preferable, and an alkyl group and a substituted alkyl group are particularly preferable. The aliphatic group may be an open-chain aliphatic group or a cyclic aliphatic group, and the open-chain aliphatic group may further be branched. Specific examples of R³¹ above include a hydrogen atom, a methyl group, an ethyl group, a butyl group, an octyl group, a hydroxy group, a methoxy group, an ethoxy group, a butoxy group, an octyloxy group, an acetyloxy group, a pivaloyloxy group, a benzoyloxy group, —O., and an acetyl group. Among them, R³¹ is more preferably a hydrogen atom.

Specific examples of Z¹ include a hydrogen atom, a hydroxy group, a methoxy group, an ethoxy group, a propyloxy group, an octyloxy group, a phenoxyethoxy group, a phenoxy group, an acetyloxy group, a propionyloxy group, a pivaloyloxy group, a benzoyloxy group, a thienyloxy group, a methoxycarbonyloxy group, a butylaminocarbonyloxy group, a phenylaminocarbonyloxy group, an amino group, an ethylamino group, a dibutylamino group, a dioctylamino group, a phenylamino group, a diphenylamino group, a hydroxyethylamino group, a bis(hydroxyethyl)amino group, a cyanoethylamino group, a carboxyethylamino group, a methoxycarbonyloxyethylamino group, a chloropropylamino group, a methylsulfonylamino group, a phenylsulfonylamino group, a butylaminocarbonylamino group, a carboxyl group, a methoxycarbonyl group, a hydroxyethyloxycarbonyl group, a chlorine atom, a fluorine atom, a bromine atom, an iodine atom, a methyl group, an ethyl group, an octadecyl group, a phenyl group, a tolyl group, an epoxy group, a chloroacetylamino group, and a chloroacetyloxy group. Specific examples of Z² include a hydrogen atom, a methyl group, an ethyl group, a butyl group, an octyl group, a phenyl group, a naphthyl group, a hydroxy group, a methoxy group, and an ethoxy group.

Z¹ and Z² may form a carbonyl group in a form containing the carbon atom substituted by Z¹ and Z², or may be bonded to each other to form a 5- to 7-membered ring. At least one of Z¹ and Z² may form a bond as a result of a reaction with a reactive group contained in a polymer, an oligomer, or a low molecular weight compound. In this case, the hindered amine compound means a compound having at least two hindered amine frameworks per molecule. In this case, at least one of Z¹ and Z² is a hydroxy group, an acyloxy group substituted with a halogen atom or a sulfonic acid ester group, an amino group, an acylamino group substituted with a halogen atom or a sulfonic acid ester group, a carboxyl group, a carbonyl group, etc., and these substituents and a polymer, an oligomer, or a low molecular weight compound having a reactive group that can undergo an addition reaction such as an SN1, SN2, etc. nucleophilic substitution reaction, a salt forming reaction, a Michael reaction, or a reaction with an epoxy compound can form a polymeric, oligomeric, or low molecular weight hindered amine compound having at least two hindered amine frameworks per molecule.

Specific examples of the hindered amine-based compound represented by Formula (II) or Formula (III) are listed below, but the present invention is not limited thereby.

In the present invention, the basic compound may be used singly or in a combination of two or more types.

The content of the basic compound is preferably 0.001 to 10 wt % relative to the total weight of the ink composition, more preferably 0.005 to 8 wt %, and yet more preferably 0.01 to 5 wt %.

It is preferable for the content of the basic compound to be in the above-mentioned range since decomposition of the N-vinyllactam can be suppressed, thus improving the ink storage stability.

Furthermore, particularly when a hindered amine-based compound is used as the basic compound, the content of the hindered amine-based compound is preferably no greater than 5 wt %. It is preferable for the amount of hindered amine-based compound added to be no greater than 5 wt % since there is little polymerization inhibition action and the sensitivity is not degraded.

(D) Colorant

Although it is not particularly necessary to form a colored image when the ink composition of the present invention is used for formation of an image area of a lithographic printing plate, etc., in order to improve the visibility of an image area that is formed or in an attempt to form a colored image using the ink composition, it may contain a colorant.

The coloring agent that can be used in the present invention is not particularly limited, but a pigment and an oil-soluble dye that have excellent weather resistance and rich color reproduction are preferable, and it may be selected from any known coloring agent such as a soluble dye. It is preferable that the colorant that can be suitably used in the ink composition or the inkjet recording ink composition of the present invention does not function as a polymerization inhibitor in a polymerization reaction, which is a curing reaction. This is because the sensitivity of the curing reaction by actinic radiation should not be degraded.

Pigment

The pigment that can be used in the present invention is not particularly limited and, for example, organic and inorganic pigments having the numbers below described in the Color Index may be used.

That is, as a red or magenta pigment, Pigment Red 3, 5, 19, 22, 31, 38, 43, 48:1, 48:2, 48:3, 48:4, 48:5, 49:1, 53:1, 57:1, 57:2, 58:4, 63:1, 81, 81:1, 81:2, 81:3, 81:4, 88, 104, 108, 112, 122, 123, 144, 146, 149, 166, 168, 169, 170, 177, 178, 179, 184, 185, 208, 216, 226, or 257, Pigment Violet 3, 19, 23, 29, 30, 37, 50, or 88, and Pigment Orange 13, 16, 20, or 36;

-   -   as a blue or cyan pigment, Pigment Blue 1, 15, 15:1, 15:2, 15:3,         15:4, 15:6, 16, 17-1, 22, 27, 28, 29, 36, or 60;     -   as a green pigment, Pigment Green 7, 26, 36, or 50;     -   as a yellow pigment, Pigment Yellow 1, 3, 12, 13, 14, 17, 34,         35, 37, 55, 74, 81, 83, 93, 94, 95, 97, 108, 109, 110, 137, 138,         139, 153, 154, 155, 157, 166, 167, 168, 180, 185, or 193;     -   as a black pigment, Pigment Black 7, 28, or 26;     -   as a white pigment, Pigment White 6, 18, or 21, etc. may be used         according to the intended application.         Oil-Soluble Dye

The oil-soluble dye that can be used in the present invention is explained below.

The oil-soluble dye that can be used in the present invention means a dye that is substantially insoluble in water. Specifically, the solubility in water at 25° C. (the mass of dye that can be dissolved in 100 g of water) is no greater than 1 g, preferably no greater than 0.5 g, and more preferably no greater than 0.1 g. Therefore, the oil-soluble dye means a so-called water-insoluble pigment or an oil-soluble dye, and among these the oil-soluble dye is preferable.

Among the oil-soluble dyes that can be used in the present invention, as a yellow dye, any may be used. Examples thereof include aryl or heteryl azo dyes having a coupling component such as a phenol, a naphthol, an aniline, a pyrazolone, a pyridone, or an open-chain active methylene compound; azomethine dyes having a coupling component such as an open-chain active methylene compound; methine dyes such as benzylidene dyes and monomethineoxonol dyes; quinone dyes such as naphthoquinone dyes and anthraquinone dyes; and other dye species such as quinophthalone dyes, nitro/nitroso dyes, acridine dyes, and acridinone dyes.

Among the above-mentioned oil-soluble dyes that can be used in the present invention, as a magenta dye, any may be used. Examples thereof include aryl or heteryl azo dyes having a coupling component such as a phenol, a naphthol, or an aniline; azomethine dyes having a coupling component such as a pyrazolone or a pyrazolotriazole; methine dyes such as arylidene dyes, styryl dyes, merocyanine dyes, and oxonol dyes; carbonium dyes such as diphenylmethane dyes, triphenylmethane dyes, and xanthene dyes; quinone dyes such as naphthoquinones, anthraquinones, or anthrapyridones; and condensed polycyclic dyes such as dioxazine dyes.

Among the oil-soluble dyes that can be used in the present invention, as a cyan dye, any may be used. Examples thereof include indoaniline dyes, indophenol dyes, and azomethine dyes having a coupling component such as a pyrrolotriazole; polymethine dyes such as cyanine dyes, oxonol dyes, and merocyanine dyes; carbonium dyes such as diphenylmethane dyes, triphenylmethane dyes, and xanthene dyes; phthalocyanine dyes; anthraquinone dyes; aryl or heteryl azo dyes having a coupling component such as a phenol, a naphthol, or an aniline; and indigo/thioindigo dyes.

The above-mentioned dyes may be dyes that exhibit respective colors of yellow, magenta, and cyan only after a part of the chromophore dissociates, and in that case the counter cation may be an inorganic cation such as an alkali metal or ammonium, may be an organic cation such as pyridinium or a quaternary ammonium salt, or may be a polymer cation having the above cation as a partial structure.

Although not limited to the following, preferred specific examples thereof include CI Solvent Black 3, 7, 27, 29, and 34; CI Solvent Yellow 14, 16, 19, 29, 30, 56, 82, 93, and 162; CI Solvent Red 1, 3, 8, 18, 24, 27, 43, 49, 51, 72, 73, 109, 122, 132, and 218; CI Solvent Violet 3; CI Solvent Blue 2, 11, 25, 35, 38, 67, and 70; CI Solvent Green 3 and 7; and CI Solvent Orange 2.

Particularly preferred examples thereof include Nubian Black PC-0850, Oil Black HBB, Oil Yellow 129, Oil Yellow 105, Oil Pink 312, Oil Red 5B, Oil Scarlet 308, Vali Fast Blue 2606, Oil Blue BOS (manufactured by Orient Chemical Industries, Ltd.), Aizen Spilon Blue GNH (manufactured by Hodogaya Chemical Co., Ltd.), Neopen Yellow 075, Neopen Magenta SE1378, Neopen Blue 808, Neopen Blue FF4012, and Neopen Cyan FF4238 (manufactured by BASF).

In the present invention, the oil-soluble dye may be used singly or in a combination of two or more types.

Furthermore, when the oil soluble dye is used as a colorant, another colorant such as a water-soluble dye, a disperse dye, or a pigment may be contained as necessary in a range that does not interfere with the effects of the present invention.

In the present invention, a disperse dye may be used in a range that enables it to be dissolved in a water-immiscible organic solvent. Disperse dyes generally include water-soluble dyes, but in the present invention it is preferable for the disperse dye to be used in a range such that it dissolves in a water-immiscible organic solvent. Specific preferred examples of the disperse dye include CI Disperse Yellow 5, 42, 54, 64, 79, 82, 83, 93, 99, 100, 119, 122, 124, 126, 160, 184:1, 186, 198, 199, 201, 204, 224, and 237; CI Disperse Orange 13, 29, 31:1, 33, 49, 54, 55, 66, 73, 118, 119, and 163; CI Disperse Red 54, 60, 72, 73, 86, 88, 91, 92, 93, 111, 126, 127, 134, 135, 143, 145, 152, 153, 154, 159, 164, 167:1, 177, 181, 204, 206, 207, 221, 239, 240, 258, 277, 278, 283, 311, 323, 343, 348, 356, and 362; CI Disperse Violet 33; CI Disperse Blue 56, 60, 73, 87, 113, 128, 143, 148, 154, 158, 165, 165:1, 165:2, 176, 183, 185, 197, 198, 201, 214, 224, 225, 257, 266, 267, 287, 354, 358, 365, and 368; and CI Disperse Green 6:1 and 9.

The coloring agent that can be used in the present invention is preferably added to the ink composition or the inkjet recording ink composition of the present invention and then dispersed in the ink to an appropriate degree. For dispersion of the coloring agent, for example, a dispersing machine such as a ball mill, a sand mill, an attritor, a roll mill, an agitator, a Henschel mixer, a colloidal mill, an ultrasonic homogenizer, a pearl mill, a wet type jet mill, or a paint shaker may be used.

The coloring agent may be added directly to the ink composition of the present invention, but in order to improve dispersibility it may be added in advance to a solvent or a dispersing medium such as a radically polymerizable compound used in the present invention.

In the present invention, in order to avoid the problem of the solvent resistance being degraded when the solvent remains in the cured image and the VOC (Volatile Organic Compound) problem of the residual solvent, it is preferable to add the coloring agent in advance to a dispersing medium such as a radically polymerizable compound. As a polymerizable compound used, it is preferable in terms of dispersion suitability to select a monomer having the lowest viscosity.

These colorants may be used by appropriately selecting one type or two or more types according to the intended purpose of the ink composition.

When a colorant such as a pigment that is present as a solid in the ink composition of the present invention is used, it is preferable for the colorant, the dispersant, the dispersing medium, dispersion conditions, and filtration conditions to be set so that the average particle size of colorant particles is preferably 0.005 to 0.5 μm, more preferably 0.01 to 0.45 μm, and yet more preferably 0.015 to 0.4 μm. By such control of particle size, clogging of a head nozzle can be suppressed, and the ink storage stability, the ink transparency, and the curing sensitivity can be maintained.

The content of the colorant in the ink composition of the present invention is appropriately selected according to the color and the intended purpose, and is generally preferably 0.01 to 30 wt % relative to the weight of the entire ink composition.

(E) Dispersant

It is preferable to add a dispersant when dispersing the colorant. The type of dispersant is not particularly limited, but it is preferable to use a polymeric dispersant. The polymeric dispersant referred to in the present invention means a dispersant having a molecular weght (a weight-average molecular weight for one having a molecular weight distribution) of 1,000 or greater. The polymeric despersant preferably has a molecular weight of 1,000 to 100,000, more preferably 2,000 to 50,000 and particularly preferably 3,000 to 20,000.

Furthermore, in the present invention, it is particularly preferable to use a basic dispersant as the polymeric dispersant. As the basic polymeric dispersant, a known polymeric dispersant having a basic functional group may broadly be used.

Examples of the basic functional group include a primary, secondary, or tertiary amino group, and a nitrogen-containing hetero ring such as pyridine, pyrimidine, or pyrazine. A fatty acid amine-based dispersant may particularly preferably be used.

In the present invention, it is particularly preferable for the dispersant to be a compound having an amine value that is larger than its acid value.

The amine value referred to here means the total amount of primary, secondary, and tertiary amines, and is expressed as the number of mg of KOH equivalent to the hydrochloric acid necessary for neutralizing 1 g of a sample. The acid value referred to here is expressed as the number of mg of KOH necessary for neutralizing a free fatty acid, a resin acid, etc. contained in 1 g of a sample.

In the present invention, the difference between the acid value and the amine value of the dispersant is preferably at least, 5 mg KOH/g.

Specific examples of the basic polymeric dispersant include Disper BYK-161 (amine value 11 mg KOH/g, acid value 0 mg KOH/g), Disper BYK-162 (amine value 13 mg KOH/g, acid value 0 mg KOH/g), Disper BYK-163 (amine value 10 mg KOH/g, acid value 0 mg KOH/g), Disper BYK-164 (amine value 18 mg KOH/g, acid value 0 mg KOH/g), Disper BYK-166 (amine value 20 mg KOH/g, acid value 0 mg KOH/g), Disper BYK-167 (amine value 13 mg KOH/g, acid value 0 mg KOH/g), Disper BYK-168 (amine value 10 mg KOH/g, acid value 0 mg KOH/g), and Disper BYK-182 (amine value 13 mg KOH/g, acid value 0 mg KOH/g) (all manufactured by BYK Chemie), EFKA4046 (amine value 17 to 21 mg KOH/g, acid value 0 mg KOH/g), EFKA4060 (amine value 6 to 10 mg KOH/g, acid value 0 mg KOH/g), EFKA4080 (amine value 3.6 to 4.1 mg KOH/g, acid value 0 mg KOH/g), EFKA4800 (amine value 37 to 43 mg KOH/g, acid value 0 mg KOH/g), and EFKA7462 (amine value 8 mg KOH/g, acid value 0 mg KOH/g) (all manufactured by EFKA Additives), various types of Solsperse dispersant such as Solsperse 13240 (basic dispersant), Solsperse 13940 (basic dispersant), Solsperse 24000 (amine value 47 mg KOH/g, acid value 24 mg KOH/g), Soisperse 28000 (basic dispersant), and Solsperse 32000 (amine value 27.1 mg KOH/g, acid value 24.8 mg KOH/g) (all manufactured by Zeneca), and Disparlon DA-234 (amine value 20 mg KOH/g, acid value 16 mg KOH/g) and Disparlon DA-325 (amine value 20 mg KOH/g, acid value 14 mg KOH/g) (both manufactured by Kusumoto Chemicals, Ltd.).

The basic dispersant may be used on its own as the basic compound (C), but it is preferably used in combination with the basic compound (C). In this case, the stability of the N-vinyllactam improves, and the long-term storage stability of the ink composition improves, which is preferable.

Another known dispersant may also be used. Specific examples thereof include polymeric dispersants such as Disper BYK-101, Disper BYK-102, Disper BYK-103, Disper BYK-106, Disper BYK-111, Disper BYK-170, Disper BYK-171, and Disper BYK-174 (all manufactured by BYK Chemie), and EFKA4010 and EFKA5010 (both manufactured by EFKA Additives); various types of Solsperse dispersants such as Solsperse 3000, 26000, 36000, 39000, 41000, and 71000 (all manufactured by Zeneca); Adeka Pluronic L31, F38, L42, L44, L61, L64, F68, L72, P95, F77, P84, F87, P94, L101, P103, F108, L121, and P-123 (manufactured by Adeka Corporation), and Disparlon KS-860 and 873SN (polymeric dispersants), #2150 (aliphatic polyvalent carboxylic acid), and #7004 (polyether ester type) (all manufactured by Kusumoto Chemicals, Ltd.).

It is also possible to use in combination a pigment derivative such as a phthalocyanine derivative (product name: EFKA-745 (manufactured by EFKA)), or Solsperse 5000, 12000, or Solsperse 22000 (manufactured by Zeneca).

The content of the dispersant in the ink composition of the present invention is appropriately selected according to the intended purpose, but in general it is preferably 0.01 to 5 wt % of the weight of the entire ink composition.

(F) Surfactant

It is preferable to add a surfactant to the ink composition of the present invention in order to impart long-term discharge stability.

As the surfactant, those described in JP-A-62-173463 and 62-183457 can be cited. Examples thereof include anionic surfactants such as dialkylsulfosuccinic acid salts, alkylnaphthalene sulfonic acid salts, and fatty acid salts, nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl aryl ethers, acetylene glycols, and polyoxyethylenetpolyoxypropylene block copolymers, and cationic surfactants such as alkylamine salts and quaternary ammonium salts. An organofluoro compound may be used instead of the above-mentioned surfactant. The organofluoro compound is preferably hydrophobic. Examples of the organofluoro compound include fluorine-based surfactants, oil-like fluorine-based compounds (e.g. fluorine oil), solid fluorine compound resins (e.g. tetrafluoroethylene resin), and those described in JP-B-57-9053 (paragraphs 8 to 17) and JP-A-62-135826.

The content of the surfactant in the ink composition of the present invention is appropriately selected according to the intended purpose and is generally preferably 0.0001 to 1 wt % relative to the weight of the entire ink composition.

Other Component

The ink composition of the present invention may comprise another component as necessary. Examples of the other component include a sensitizing colorant, a cosensitizer, a UV absorber, an antioxidant, an antifading agent, a conductive salt, a solvent, and a polymer compound.

Sensitizing Dye

The ink composition of the present invention may contain a sensitizing dye in order to promote decomposition of the above-mentioned polymerization initiator by absorbing specific actinic radiation, in particular when used for inkjet recording. The sensitizing dye absorbs specific actinic radiation and attains an electronically excited state. The sensitizing dye in the electronically excited state causes actions such as electron transfer, energy transfer, or heat generation upon contact with the polymerization initiator. This causes the polymerization initiator to undergo a chemical change and decompose, thus forming a radical, an acid, or a base.

Preferred examples of the sensitizing dye include those that belong to compounds below and have an adsorption wavelength in the region of 350 nm to 450 nm.

Polynuclear aromatic compounds (e.g. pyrene, perylene, triphenylene), xanthenes (e.g. fluorescein, eosin, erythrosine, rhodamine B, rose bengal), cyanines (e.g. thiacarbocyanine, oxacarbocyanine), merocyanines (e.g. merocyanine, carbomerocyanine), thiazines (e.g. thionine, methylene blue, toluidine blue), acridines (e.g. acridine orange, chloroflavin, acriflavine), anthraquinones (e.g. anthraquinone), squaryliums (e.g. squarylium), and coumarins (e.g. 7-diethylamino-4-methylcoumarin).

Preferred examples of the sensitizing dye include compounds represented by Formulae (IX) to (XIII) below.

In Formula (IX), A¹ denotes a sulfur atom or NR⁵⁰, R⁵⁰ denotes an alkyl group or an aryl group, L² denotes a non-metallic atomic group forming a basic nucleus of a dye in cooperation with a neighboring A¹ and the neighboring carbon atom, R⁵¹ and R⁵² independently denote a hydrogen atom or a monovalent non-metallic atomic group, and R⁵¹ and R⁵² may be bonded together to form an acidic nucleus of a dye. W denotes an oxygen atom or a sulfur atom.

In Formula (X), Ar¹ and Ar² independently denote an aryl group and are connected to each other via a bond of -L³-. Here, L³ denotes —O— or —S—. W has the same meaning as that shown in Formula (IX).

In Formula (XI), A₂ denotes a sulfur atom or NR⁵⁹, L⁴ denotes a non-metallic atomic group forming a basic nucleus of a dye in cooperation with the neighboring A₂ and carbon atom, R⁵³, R⁵⁴, R⁵⁵, R⁵⁶, R⁵⁷, and R⁵⁸ independently denote a monovalent non-metallic atomic group, and R⁵⁹ denotes an alkyl group or an aryl group.

In Formula (XII), A³ and A⁴ independently denote —S—, —NR⁶²—, or —NR⁶³—, R⁶² and R⁶³ independently denote a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aryl group, L⁵ and L⁶ independently denote a non-metallic atomic group forming a basic nucleus of a dye in cooperation with the neighboring A³ and A⁴ and neighboring carbon atom, and R⁶⁰ and R⁶¹ independently denote a hydrogen atom or a monovalent non-metallic atomic group, or are bonded to each other to form an aliphatic or aromatic ring.

In Formula (XIII), R⁶⁶ denotes an aromatic ring or a hetero ring, which may have a substituent, and A⁵ denotes an oxygen atom, a sulfur atom, or —NR⁶⁷—. R⁶⁴, R⁶⁵, and R⁶⁷ independently denote a hydrogen atom or a monovalent non-metallic atomic group, and R⁶⁷ and R⁶⁴, and R⁶⁵ and R⁶⁷ may be bonded to each other to form an aliphatic or aromatic ring.

Specific examples of the compounds represented by Formulae (IX) to (XIII) include (E-1) to (E-20) listed below.

The content of the sensitizing colorant in the ink composition of the present invention is appropriately selected according to the intended purpose, but it is generally preferably 0.05 to 4 wt % relative to the weight of the entire ink composition.

Cosensitizer

The ink composition of the present invention preferably comprises a cosensitizer. In the present invention, the cosensitizer has the function of further improving the sensitivity of the sensitizing dye to actinic radiation or the function of suppressing inhibition by oxygen of polymerization of a polyrnerizable compound, etc.

Examples of such a cosensitizer include amines such as compounds described in M. R. Sander et al., ‘Journal of Polymer Society’, Vol. 10, p. 3173 (1972), JP-B-44-20189, JP-A-51-82102, JP-A-52-134692, JP-A-59-138205, JP-A-60-84305, JP-A-62-18537, JP-A-64-33104, and Research Disclosure No. 33825, and specific examples thereof include triethanolamine, ethyl p-dimethylaminobenzoate, p-formyldimethylaniline, and p-methylthiodimethylaniline.

Other examples of the cosensitizer include thiols and sulfides such as thiol compounds described in JP-A-53-702, JP-B-55-500806, and JP-A-5-142772, and disulfide compounds of JP-A-56-75643, and specific examples thereof include 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptobenzimidazole, 2-mercapto-4(3H)-quinazoline, and β-mercaptonaphthalene.

Yet other examples of the cosensitizer include amino acid compounds (e.g. N-phenylglycine, etc.), organometallic compounds described in JP-B-48-42965 (e.g. tributyltin acetate, etc.), hydrogen-donating compounds described in JP-B-55-34414, sulfur compounds described in JP-A-6-308727 (e.g. trithiane, etc.), and phosphorus compounds described in JP-A-6-250387 (diethylphosphite, etc.).

The content of the cosensitizer in the ink composition of the present invention is appropriately selected according to the intended purpose, but it is generally preferably 0.05 to 4 wt % relative to the weight of the entire ink composition.

UV Absorber

A UV absorber may be used from the viewpoint of improving the weather resistance of an image obtained and preventing discoloration.

The UV absorbers include benzotriazole compounds described in JP-A-58-185677, JP-A-61-190537, JP-A-2-782, JP-A-5-197075 and JP-A-9-34057; benzophenone compounds described in JP-A-46-2784, JP-A-5-194483 and U.S. Pat. No. 3,214,463; cinnamic acid compounds described in JP-B-48-30492, JP-B-56-21141 and JP-A-10-88106; triazine compounds described in JP-A-4-298503, JP-A-8-53427, JP-A-8-239368, JP-A-10-182621 and JP-W-8-501291 (the term “JP-W” as used herein means an unexamined published international patent application); compounds described in Research Disclosure No. 24239; and compounds represented by stilbene and benzoxazole compounds, which absorb ultraviolet rays to emit fluorescence, the so-called fluorescent brightening agents.

The amount thereof added is appropriately selected according to the intended application, and it is generally on the order of 0.5 to 15 wt % on the basis of the solids content in the ink composition.

Antioxidant

In order to improve the stability of the ink composition, an antioxidant may be added. Examples of the antioxidant include those described in Laid-open European Patent Nos. 223739, 309401, 309402, 310551, 310552, and 459416, Laid-open German Patent No. 3435443, JP-A-54-48535, JP-A-62-262047, JP-A-63-113536, JP-A-63-163351, JP-A-2-262654, JP-A-2-71262, JP-A-3-121449, JP-A-5-61166, JP-A-5-119449, and U.S. Pat. Nos. 4,814,262 and 4,980,275.

The amount thereof added is appropriately selected according to the intended application, and it is preferably on the order of 0.1 to 8 wt % on the basis of the solids content in the ink composition.

Antifading Agent

The ink composition of the present invention may employ various organic and metal complex antifading agents. The organic antifading agents include hydroquinones, alkoxyphenols, dialkoxyphenols, phenols, anilines, amines, indanes, chromans, alkoxyanilines, and heterocycles, and the metal complex antifading agents include nickel complexes and zinc complexes. More specifically, there can be used compounds described in patents cited in Research Disclosure, No. 17643, Items VII-I to J, ibid., No. 15162, ibid., No. 18716, page 650, left-hand column, ibid., No. 36544, page 527, ibid., No. 307105, page 872, and ibid., No. 15162, and compounds contained in general formulae and compound examples of typical compounds described in JP-A-62-21572, pages 127 to 137.

The amount thereof added is appropriately selected according to the intended application, and it is preferably on the order of 0.1 to 8 wt % on the basis of the solids content in the ink composition.

Conductive Salt

The ink composition of the present invention may contain, for the purpose of controlling discharge properties, a conductive salt such as potassium thiocyanate, lithium nitrate, ammonium thiocyanate, or dimethylamine hydrochloride.

Solvent

It is also effective to add a trace amount of organic solvent to the ink composition of the present invention in order to improve the adhesion to a recording medium.

Examples of the solvent include ketone-based solvents such as acetone, methyl ethyl ketone, and diethyl ketone, alcohol-based solvents such as methanol, ethanol, 2-propanol, 1-propanol, 1-butanol, and tert-butanol, chlorine-based solvents such as chloroform and methylene chloride, aromatic-based solvents such as benzene and toluene, ester-based solvents such as ethyl acetate, butyl acetate, and isopropyl acetate, ether-based solvents such as diethyl ether, tetrahydrofuran, and dioxane, and glycol ether-based solvents such as ethylene glycol monomethyl ether and ethylene glycol dimethyl ether.

In this case, it is effective if the amount thereof added is in a range that does not cause problems with the solvent resistance or the VOC, and the amount is preferably in the range of 0.1 to 5 wt % relative to the total amount of the ink composition, and more preferably 0.1 to 3 wt %.

High Molecular Weight Compound

The ink composition may contain various types of high molecular weight compounds in order to adjust film physical properties. Examples of the high molecular weight compounds include acrylic polymers, polyvinylbutyral resins, polyurethane resins, polyamide resins, polyester resins, epoxy resins, phenol resins, polycarbonate resins, polyvinylbutyral resins, polyvinylformal resins, shellac, vinylic resins, acrylic resins, rubber-based resins, waxes, and other natural resins. They may be used in a combination of two or more types. Among these, a vinylic copolymer obtained by copolymerization of an acrylic monomer is preferable. Furthermore, as a copolymer component of the high molecular weight compound, a copolymer containing as a structural unit a ‘carboxyl group-containing monomer’, an ‘alkyl methacrylate ester’, or an ‘alkyl acrylate ester’ may preferably be used.

In addition to the above, the composition may contain as necessary, for example, a leveling additive, a matting agent, a wax for adjusting film physical properties, or a tackifier in order to improve the adhesion to a recording medium such as polyolefin or PET, the tackifier not inhibiting polymerization.

Specific examples of the tackifier include high molecular weight tacky polymers described on pp. 5 and 6 of JP-A-2001-49200 (e.g. a copolymer formed from an ester of (meth)acrylic acid and an alcohol having an alkyl group with 1 to 20 carbons, an ester of (meth)acrylic acid and an alicyclic alcohol having 3 to 14 carbons, or an ester of (meth)acrylic acid and an aromatic alcohol having 6 to 14 carbons), and a low molecular weight tackifying resin having a polymerizable unsaturated bond.

Properties of Ink Composition

In the present invention, the ink composition preferably has a viscosity at 25° C. of no more than 50 mPa·s, and more preferably no more than 40 mPa·s. The viscosity is preferably 3 to 50 mPa·s, more preferably 5 to 40 mPa·s, and particularly preferably 7 to 30 mPa·s. Furthermore, the viscosity of the ink composition at the discharge temperature (e.g. 25° C. to 80° C., and preferably 25° C. to 50° C.) is preferably 3 to 15 mPa·s, and more preferably 3 to 13 mPa·s. With regard to the ink composition of the present invention, it is preferable that its omponent ratio is appropriately adjusted so that the viscosity is in the above-mentioned range. When the viscosity at room temperature is set to be high, even when a porous recording medium is used, penetration of the ink into the recording medium can be prevented, uncured monomer can be reduced, and the odor can be reduced. Furthermore, ink spreading when ink droplets have landed can be suppressed, and as a result there is the advantage that the image quality is improved.

The surface tension of the ink composition of the present invention at 25° C. is preferably 20 to 35 mN/m, and yet more preferably 23 to 33 mN/m. When recording is carried out on various types of recording medium such as polyolefin, PET, coated paper, and uncoated paper, from the viewpoint of spread and penetration, it is preferably at least 20 mN/m, and from the viewpoint of wettability it is preferably not more than 35 mN/m.

(2) Inkjet Recording Method and Inkjet Recording Device

The ink composition of the present invention is used for inkjet recording.

The inkjet recording method of the present invention is a method for forming an image by discharging the ink composition of the present invention onto a recording medium (support, recording material, etc.) for inkjet recording and curing the ink by irradiating the ink composition so discharged onto the recording medium with actinic radiation.

More particularly, the inkjet recording method of the present invention comprises (a¹) a step of discharging the ink composition of the present invention onto a recording medium and (b¹) a step of curing the ink composition by irradiating the ink composition so discharged with actinic radiation,

The inkjet recording method of the present invention comprises the steps (a¹) and (b¹) above and thus forms an image from the ink composition cured on the recording medium.

The step (a¹) of the inkjet recording method of the present invention may employ an inkjet recording device that will be described in detail below.

Inkjet Recording Device

An inkjet recording device used in the inkjet recording method of the present invention is not particularly limited, and any known inkjet recording device that can achieve an intended resolution may be used. That is, any known inkjet recording device, such as a commercial product, may be used in order to discharge an ink onto a recording medium in step (a¹) of the inkjet recording method of the present invention.

The inkjet recording device that can be used in the present invention is equipped with, for example, an ink supply system, a temperature sensor, and an actinic radiation source.

The ink supply comprises, for example, a main tank containing the ink composition of the present invention, a supply pipe, an ink supply tank immediately before an inkjet head, a filter, and a piezo system inkjet head. The piezo system inkjet head may be driven so as to discharge a multisize dot of 1 to 100 pL, and preferably 8 to 30 pL, at a resolution of 320×320 to 4,000×4,000 dpi, preferably 400×400 to 1,600×1,600 dpi, and more preferably 720×720 dpi. Here, dpi referred to in the present invention means the number of dots per 2.54 cm.

As described above, since it is desirable for the radiation curing type ink to be discharged at a constant temperature, a section from the ink supply tank to the inkjet head is thermally insulated and heated. A method of controlling temperature is not particularly limited, but it is preferable to provide, for example, temperature sensors at a plurality of pipe section positions, and control heating according to the ink flow rate and the temperature of the surroundings. The temperature sensors may be provided on the ink supply tank and in the vicinity of the inkjet head nozzle. Furthermore, the head unit that is to be heated is preferably thermally shielded or insulated so that the device main body is not influenced by the temperature of the outside air. In order to reduce the printer start-up time required for heating, or in order to reduce the thermal energy loss, it is preferable to thermally insulate the head unit from other sections and also to reduce the heat capacity of the entire heated unit.

When the ink composition or the inkjet recording ink composition of the present invention is discharged using the above mentioned inkjet recording device, the ink composition is preferably discharged after being heated to preferably 25° C. to 80° C., and more preferably 25° C. to 50° C., so as to reduce the viscosity of the ink composition to preferably 3 to 15 mPa·s, and more preferably 3 to 13 mPa·s. In particular, it is preferable to use the ink composition having an ink viscosity at 25° C. of no more than 50 mPa·s since a good discharge stability can be obtained. By employing this method, high discharge stability can be realized.

The radiation curing type ink composition such as the ink composition of the present invention generally has a viscosity that is higher than that of a normal ink composition or a water-based ink used for an inkjet recording ink, and variation in viscosity due to a change in temperature at the time of discharge is large. Viscosity variation in the ink has a large effect on changes in liquid droplet size and changes in liquid droplet discharge speed and, consequently, causes the image quality to be degraded. It is therefore necessary to maintain the ink discharge temperature as constant as possible. In the present invention, the control range for the temperature is desirably ±5° C. of a set temperature, preferably ±2° C. of the set temperature, and more preferably ±1° C. of the set temperature.

The step (b¹) of curing the discharged ink composition by irradiating the ink composition with actinic radiation is now explained.

The ink composition discharged onto the recording medium cures upon exposure to actinic radiation. This is due to an initiating species such as a radical, an acid, or a base being generated by decomposition of the polymerization initiator contained in the ink composition of the present invention by irradiation with actinic radiation, the initiating species functioning so as to make a polymerization reaction of a radically polymerizable compound take place and to promote it. In this process, if a sensitizing colorant is present together with the polymerization initiator in the ink composition, the sensitizing colorant in the system absorbs actinic radiation, becomes excited, and promotes decomposition of the polymerization initiator by contact with the polymerization initiator, thus enabling a curing reaction with higher sensitivity to be achieved.

The actinic radiation used in this process may include α rays, γ rays, an electron beam, X rays, UV rays, visible light, and IR rays. Although it depends on the absorption characteristics of the sensitizing dye, the peak wavelength of the actinic radiation is preferably 200 to 600 nm, more preferably 300 to 450 nm, and particularly preferably 350 to 420 nm.

Furthermore, in the present invention, the polymerization initiation system has sufficient sensitivity for low output actinic radiation. The actinic radiation is applied therefore so that the illumination intensity on the exposed surface is, for example, 10 to 4,000 mW/cm², and preferably 20 to 2,500 mW/cm².

As an actinic radiation source, a mercury lamp, a gas/solid laser, etc. are mainly used, and for UV photocuring inkjet a mercury lamp and a metal halide lamp are widely known. However, from the viewpoint of protection of the environment, there has recently been a strong desire for mercury not to be used, and replacement by a GaN semiconductor UV light emitting device is very useful from industrial and environmental viewpoints. Furthermore, LEDs (UV-LED) and LDs (UV-LD) have small dimensions, long life, high efficiency, and low cost, and their use as a photocuring inkjet light source can be expected.

Furthermore, light-emitting diodes (LED) and laser diodes (LD) may be used as the source of actinic radiation. In particular, when a UV ray source is needed, a UV-LED or a UV-LD may be used. For example, Nichia Corporation has marketed a violet LED having a wavelength of the main emission spectrum of between 365 nm and 420 nm. Furthermore, when a shorter wavelength is needed, U.S. Pat. No. 6,084,250 discloses an LED that can emit actinic radiation whose wavelength is centered between 300 nm and 370 nm. Furthermore, another UV-LED is available, and irradiation can be carried out with radiation of a different UV bandwidth. The actinic radiation source particularly preferable in the present invention is a UV-LED, and a UV-LED having a peak wavelength at 350 to 420 nm is particularly preferable.

The maximum illumination intensity of the LED on a recording medium is preferably 10 to 2,000 mW/cm², more preferably 20 to 1,000 mW/cm², and particularly preferably 50 to 800 mJ/cm².

The ink composition of the present invention is desirably exposed to such actinic radiation for, for example, 0.01 to 120 sec., and preferably 0.1 to 90 sec.

Irradiation conditions and a basic method for irradiation with actinic radiation are disclosed in JP-A-60-132767. Specifically, a light source is provided on either side of a head unit that includes an ink discharge device, and the head unit and the light source are made to scan by a so-called shuttle system. Irradiation with actinic radiation is carried out after a certain time (e.g. 0.01 to 0.5 sec., preferably 0.01 to 0.3 sec., and more preferably 0.01 to 0.15 sec.) has elapsed from when the ink has landed. By controlling the time from ink landing to irradiation so as to be a minimum in this way, it becomes possible to prevent the ink that has landed on a recording medium from spreading before being cured. Furthermore, since the ink can be exposed before it reaches a deep area of a porous recording medium that the light source cannot reach, it is possible to prevent monomer from remaining unreacted, and as a result the odor can be reduced.

Furthermore, curing may be completed using another light source that is not driven. WO99/54415 discloses, as an irradiation method, a method employing an optical fiber and a method in which a collimated light source is incident on a mirror surface provided on a head unit side face, and a recorded area is irradiated with UV light.

By employing such a recording method, it is possible to maintain a uniform dot diameter for landed ink even for various types of recording media having different surface wettability, thereby improving the image quality. In order to obtain a color image, it is preferable to superimpose colors in order from those with a low lightness. By superimposing inks in order from one with low lightness, it is easy for radiation to reach a lower ink, the curing sensitivity is good, the amount of residual monomer decreases, odor is reduced, and an improvement in adhesion can be expected. Furthermore, although it is possible to discharge all colors and then expose them at the same time, it is preferable to expose one color at a time from the viewpoint of promoting curing.

In this way, the ink composition of the present invention is cured by irradiation with actinic radiation in high sensitivity to thus form an image on the surface of the recording medium.

(3) Lithographic Printing Plate and Production Process

It is possible to produce a lithographic printing plate by applying the ink composition of the present invention to a hydrophilic support by the inkjet recording method of the present invention and curing it.

A process for producing a lithographic printing plate by employing the inkjet recording method of the present invention (the process for producing a lithographic printing plate of the present invention) and a lithographic printing plate obtained thereby (the lithographic printing plate of the present invention) are explained below.

The lithographic printing plate of the present invention comprises a hydrophilic support and a hydrophobic image formed on the hydrophilic support. This process for producing a lithographic printing plate comprises the following steps.

(a²) a step of discharging the ink composition of the present invention onto a hydrophilic support, and

(b²) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation so as to form a hydrophobic image on the hydrophilic support by curing the ink composition.

That is, a lithographic printing plate can be produced in the same manner as in the inkjet recording method of the present invention except that a support having a hydrophilic surface that is suitable as a lithographic printing plate support is used as a recording medium.

Conventionally, a lithographic printing plate has been produced by imagewise exposing to light a so-called PS plate having an arrangement in which a lipophilic photosensitive resin layer is provided on a hydrophilic support as described above so as to solubilize or cure the exposed area and form an image, followed by dissolving and removing a non-image area.

On the other hand, the lithographic printing plate of the present invention can be formed by employing the process for producing a lithographic printing plate of the present invention (the inkjet recording method of the present invention) so as to discharge an ink composition directly onto the surface of a hydrophilic support in accordance with digitized image information and cure it to form a hydrophobic image area. This enables a lithographic printing plate to be prepared more easily than by the conventional method.

Hydrophilic Support Used for Lithographic Printing Plate

The lithographic printing plate of the present invention comprises a hydrophilic support and an hydrophobic image formed by the ink composition of the present invention on the support.

The support for lithographic printing plate (recording medium) onto which the ink composition of the present invention is discharged is not particularly limited, and a dimensionally stable sheet-form support may be used. It is preferable that a material forming the support has a hydrophilic surface from the view point of the image quality of the printed material thus obtained.

Examples of materials forming the support include paper, paper laminated with a plastic (e.g. polyethylene, polypropylene, polystyrene, etc.), a metal sheet (e.g. aluminum, zinc, copper, etc.), a plastic film (e.g. cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate butyrate, cellulose nitrate, polyethylene terephthalate, polyethylene, polystyrene, polypropylene, polycarbonate, polyvinyl acetal, etc.), and paper or plastic film on which the above-mentioned metal is laminated or vapor-deposited. Preferred examples of the support include a polyester film and aluminum sheet. Among these, aluminum sheet is particularly preferable since the dimensional stability is good and it is relatively inexpensive.

The aluminum sheet is a pure aluminum sheet, an alloy sheet containing aluminum as a main component and a small amount of a different element, or a thin film of aluminum or an aluminum alloy laminated with a plastic. Examples of the different element contained in the aluminum alloy include silicon, iron, manganese, copper, magnesium, chromium, zinc, bismuth, nickel, and titanium. The content of the different element in the alloy is preferably equal to or less than 10 wt %. In the present invention, a pure aluminum sheet is preferable, but since it is difficult to produce completely pure aluminum because of the refining technique, a trace amount of a different element may be contained. The composition of the aluminum sheet is not specified, and a known generally used material may be utilized as appropriate.

The support preferably has a thickness of 0.1 to 0.6 mm, and more preferably 0.15 to 0.4 mm.

Prior to the aluminum sheet being used, it is preferably subjected to a surface treatment such as a surface roughening treatment or an anodizing treatment. Surface treatment makes it easy to improve the hydrophilicity and ensure that there is good adhesion between an image recording layer and the support. Prior to the aluminum sheet being subjected to the surface roughening treatment, it may be subjected as desired to a degreasing treatment using a surfactant, an organic solvent, an aqueous alkaline solution, etc. in order to remove rolling oil on the surface.

The surface roughening treatment for the aluminum sheet surface may be carried out by various types of methods, and examples thereof include a mechanical surface roughening treatment, an electrochemical surface roughening treatment (a surface roughening treatment involving dissolving the surface electrochemically), and a chemical surface roughening treatment (a surface roughening treatment involving selectively dissolving the surface chemically).

As a method for the mechanical surface roughening treatment, a known method such as a ball grinding method, a brush grinding method, a blast grinding method, or a buff grinding method may be used. It is also possible to use a transfer method in which an irregular shape is transferred using a roller provided with irregularities in an aluminum rolling stage.

As a method for the electrochemical surface roughening treatment, for example, a method in which alternating current or direct current is applied in an electrolyte solution containing an acid such as hydrochloric acid or nitric acid can be cited. It is also possible to employ a method as described in JP-A-54-63902 in which a mixed acid is used.

The aluminum sheet subjected to a surface roughening treatment is subjected as necessary to an alkali etching treatment using an aqueous solution of potassium hydroxide, sodium hydroxide, etc.; furthermore, after neutralization, it may be subjected to an anodizing treatment as desired in order to improve the abrasion resistance.

As an electrolyte that may be used for the anodizing treatment of the aluminum sheet, various types of electrolytes that form a porous oxide film may be used. In general, sulfuric acid, hydrochloric acid, oxalic acid, chromic acid, or a mixed acid thereof may be used. The concentration of the electrolyte may be determined as appropriate according to the type of electrolyte.

Conditions for the anodizing treatment depend on the type of electrolyte used and cannot be specified, but in general the electrolyte solution concentration is 1 to 80 wt %, the solution temperature is 5° C. to 70° C., the current density is 5 to 60 A/dm², the voltage is 1 to 100V, and the electrolysis time is 10 sec. to 5 min. The amount of anodized film formed is preferably 1.0 to 5.0 g/m², and more preferably 1.5 to 4.0 g/m². It is preferable for it to be in this range since good plate life and good scratch resistance of a non-image area of a lithographic printing plate can be obtained.

As the support that can be used in the present invention, a substrate that has been subjected to the above-mentioned surface treatment and has an anodized film may be used as it is, but in order to further improve the adhesion to the hydrophobic image, and the hydrophilicity, the contamination resistance, etc., the substrate may appropriately be subjected as necessary to a treatment for enlarging micropores of the anodized film, a sealing treatment, or a surface hydrophilization treatment involving immersion in an aqueous solution containing a hydrophilic compound, which are described in JP-A-2001-253181 or JP-A-2001-322365. These enlarging and sealing treatments are not limited to those described therein, and any conventionally known methods may be employed.

Sealing Treatment

The sealing treatment may be vapor sealing, a treatment with an aqueous solution containing an inorganic fluorine compound such as a single treatment with fluorozirconic acid or a treatment with sodium fluoride, vapor sealing with added lithium chloride, or a sealing treatment with hot water.

Among these, the sealing treatment with an aqueous solution containing an inorganic fluorine compound, the sealing treatment with vapor, and the sealing treatment with hot water are preferable. Each thereof is explained below.

Sealing Treatment with Aqueous Solution Containing Inorganic Fluorine Compound

In the sealing treatment with an aqueous solution containing an inorganic fluorine compound, a metal fluoride can suitably be used as the inorganic fluorine compound.

Specific examples thereof include sodium fluoride, potassium fluoride, calcium fluoride, magnesium fluoride, sodium fluorozirconate, potassium fluorozirconate, sodium fluorotitanate, potassium fluorotitanate, ammonium fluorozirconate, ammonium fluorotitanate, potassium fluorotitanate, fluorozirconic acid, fluorotitanic acid, hexafluorosilicic acid, nickel fluoride, iron fluoride, fluorophosphoric acid, and ammonium fluorophosphate. Among them, sodium fluorozirconate, sodium fluorotitanate, fluorozirconic acid, and fluorotitanic acid are preferable.

The concentration of the inorganic fluorine compound in the aqueous solution is preferably at least 0.01 wt % from the viewpoint of sealing of micropores on an anodized coating being carried out sufficiently, and more preferably at least 0.05 wt %, and it is preferably no greater than 1 wt % from the viewpoint of contamination resistance, and more preferably no greater than 0.5 wt %.

The aqueous solution containing an inorganic fluorine compound preferably further contains a phosphate compound. It is preferable for a phosphate compound to be contained since the hydrophilicity of the surface of the anodized coating improves and the machine developability and the contamination resistance can be improved.

Preferred examples of the phosphate compound include phosphates of a metal such as an alkali metal or an alkaline earth metal.

Specific examples thereof include zinc phosphate, aluminum phosphate, ammonium phosphate, ammonium phosphate dibasic, ammonium dihydrogen phosphate, monopotassium phosphate, potassium dihydrogen phosphate, potassium phosphate dibasic, calcium phosphate, ammonium sodium hydrogen phosphate, magnesium hydrogen phosphate, magnesium phosphate, ferrous phosphate, ferric phosphate, sodium dihydrogen phosphate, sodium phosphate, sodium phosphate dibasic, lead phosphate, calcium dihydrogen phosphate, lithium phosphate, phosphotungstic acid, ammonium phosphotungstate, sodium phosphotungstate, ammonium phosphomolybdate, sodium phosphomolybdate, sodium phosphite, sodium tripolyphosphate, and sodium pyrophosphate. Among these, sodium dihydrogen phosphate, sodium phosphate dibasic, potassium dihydrogen phosphate, and potassium phosphate dibasic are preferable.

The combination of the inorganic fluorine compound and the phosphate compound is not particularly limited, but the aqueous solution preferably comprises at least sodium fluorozirconate as the inorganic fluorine compound and at least sodium dihydrogen phosphate as the phosphate compound.

The concentration of the phosphate compound in the aqueous solution is preferably at least 0.01 wt % from the viewpoint of improving machine developability and contamination resistance, and more preferably at least 0.1 wt %, and it is preferably no greater than 20 wt % from the viewpoint of solubility, and more preferably no greater than 5 wt %.

The proportion of each compound in the aqueous solution is not particularly limited, but the ratio by weight of the inorganic fluorine compound and the phosphate compound is preferably 1/200 to 10/1, and more preferably 1/30 to 2/1.

Furthermore, the temperature of the aqueous solution is preferably at least 20° C., and more preferably at least 40° C., and it is preferably no higher than 100° C., and more preferably no higher than 80° C.

Moreover, the pH of the aqueous solution is preferably at least 1, and more preferably at least 2, and it is preferably no greater than 11, and more preferably no greater than 5.

A method for the sealing treatment with the aqueous solution containing an inorganic fluorine compound is not particularly limited and, for example, an immersion method and a spray method may be used. They may be employed once or a plurality of times, or in a combination of two or more types.

Among these, the immersion method is preferable. When the treatment is carried out by the immersion method, the treatment time is preferably at least 1 sec., and more preferably at least 3 sec., and it is preferably no greater than 100 sec., and more preferably no greater than 20 sec.

Sealing Treatment with Steam

With regard to the sealing treatment with steam, for example, a method in which an anodized coating is contacted with steam at high pressure or normal pressure continuously or discontinuously can be cited.

The temperature of the steam is preferably at least 80° C., and more preferably at least 95° C., and it is preferably no greater than 105° C.

The pressure of the steam is preferably in the range of (atmospheric pressure−50 mmAq) to (atmospheric pressure+300 mmAq) (1.008×10⁵ to 1.043×10⁵ Pa).

Furthermore, the time for which the coating is contacted with steam is preferably at least 1 sec., and more preferably at least 3 sec., and it is preferably no greater than 100 sec., and more preferably no greater than 20 sec.

Sealing Treatment with Hot Water

With regard to the sealing treatment with hot water, for example, a method in which an aluminum plate having an anodized coating formed thereon is immersed in hot water can be cited.

The hot water may contain an inorganic salt (e.g. a phosphate) or an organic salt.

The temperature of the hot water is preferably at least 80° C., and more preferably at least 95° C., and it is preferably no greater than 100° C.

Furthermore, the time for which immersion in hot water is carried out is preferably at least 1 sec., and more preferably at least 3 sec., and it is preferably no greater than 100 sec., and more preferably no greater than 20 sec.

With regard to a hydrophilization treatment that is used in the present invention, there is an alkali metal silicate method, as disclosed in U.S. Pat. Nos. 2,714,066, 3,181,461, 3,280,734, and 3,902,734. In this method, a support is immersed in an aqueous solution of sodium silicate, etc., or subjected to electrolysis. In addition, there is a method in which a support is treated with potassium fluorozirconate, as described in JP-B-36-22063, and a method in which a support is treated with polyvinylphosphonic acid, as described in U.S. Pat. Nos. 3,276,868, 4,153,461, and 4,689,272.

In the present invention, it is preferable for the support to have a center line average roughness of 0.10 to 1.2 μm. It is preferable for it to be in this range since good adhesion to a hydrophobic recording image, good plate life, and good contamination resistance can be obtained.

(a²) Step of Discharging the Ink Composition of the Present Invention onto Hydrophilic Support

First, the ink composition of the present invention is discharged onto a hydrophilic support. This step may employ a conventionally known inkjet recording device in the same manner as in the above-mentioned inkjet recording method. Preferred ink temperature and viscosity when the ink is discharged using the inkjet recording device are the same as above and the control method therefor is also the same as above.

(b²) Step of Curing Ink Composition by Irradiating Discharged Ink Composition with Actinic Radiation so as to Form Hydrophobic Image by Curing Ink Composition

The ink composition discharged onto the surface of a hydrophilic support is cured by irradiation with actinic radiation. Details of this curing mechanism are the same as those described for the inkjet recording method. Furthermore, the actinic radiation source used for curing the ink composition and irradiation conditions therefor are also the same as those described for the inkjet recording method.

Via the above-mentioned steps, a hydrophoblic image is formed on the surface of a hydrophilic support by curing the ink composition of the present invention, thus giving a lithographic printing plate.

In this way, by producing a lithographic printing plate by application of the inkjet recording method of the present invention, the diameter of dots of ink that has landed can be maintained at a constant size even for lithographic printing plate supports having different surface wettabilities and, as a result, a hydrophobic image can be formed with good precision.

Furthermore, as described above, the ink composition of the present invention can be cured by actinic radiation with high sensitivity, and a hydrophobic region (hydrophobic image) having excellent adhesion to a support and excellent film properties can be formed.

From the above, the lithographic printing plate of the present invention has high image quality and also has excellent plate life.

Needless to say, the ink composition of the present invention not only forms an image area of such a lithographic printing plate but is also useful as a normal ink composition.

In accordance with the present invention, it is possible to provide an ink composition and an inkjet ink composition that have excellent long-term storage stability, have excellent curability toward irradiation with actinic radiation, and for which an image obtained by curing the ink has excellent flexibility and adhesion to a substrate, an inkjet recording method employing the ink composition, and a printed material obtained by the inkjet recording method.

Furthermore, in accordance with the present invention, there can be provided a process for producing a lithographic printing plate employing an ink composition that has excellent long-term storage stability and has excellent curability toward irradiation with actinic radiation.

EXAMPLES

The present invention is explained in further detail by reference to Examples and Comparative Examples. However, the present invention should not be construed as being limited to these Examples.

‘Parts’ described below means ‘parts by weight’ unless otherwise specified.

Materials used in the present invention are as follows.

IRGALITE BLUE GLVO (cyan pigment, manufactured by Ciba Specialty Chemicals (CSC))

CINQUASIA MAGENTA RT-335 D (magenta pigment, manufactured by Ciba Specialty Chemicals)

NOVOPERM YELLOW H2G (yellow pigment, manufactured by Clariant)

SPECIAL BLACK 250 (black pigment, manufactured by Ciba Specialty Chemicals)

KRONOS 2300 (white pigment, manufactured by KRONOS)

FANCRYL 512A (corresponding to compound example M-11, manufactured by Hitachi Chemical Co., Ltd.)

N-Vinylcaprolactam (manufactured by Aldrich)

Actilane 421 (propoxylated neopentyl glycol diacrylate, manufactured by Akcros)

NK ester AMP-10G (phenoxyethyl acrylate, manufactured by Shin-Nakamura Chemical Co., Ltd.)

FIRSTCURE ST-1 (polymerization inhibitor, manufactured by ChemFirst)

Lucirin TPO (photopolymerization initiator, manufactured by BASF)

Benzophenone (photopolymerization initiator, manufactured by Wako Pure Chemical Industries, Ltd.)

IRGACURE 184 (photopolymerization initiator, manufactured by Ciba Specialty Chemicals)

BYK-307 (surfactant, manufactured by BYK Chemie)

Disper BYK-168 (dispersant, amine value 10 mg KOH/g, acid value 0 mg KOH/g, manufactured by BYK Chemie)

FIRSTCURE ITX (sensitizer, manufactured by ChemFirst)

Preparation of Cyan Mill Base A

(D) IRGALITE BLUE GLVO 300 parts by weight Actilane 421 (acrylate monomer, 500 parts by weight manufactured by Akcros) (E) Disper BYK-168 200 parts by weight

The components above were mixed by stirring to give a pigment mill base. Preparation of the pigment mill base was carried out by putting the pigment ink into an M50 disperser motor mill (manufactured by Eiger) and using zirconia beads having a diameter of 0.65 mm at a peripheral speed of 9 m/s for 4 hours.

Preparation of Magenta Mill Base B

(D) CINQUASIA MAGENTA RT-335 D 300 parts by weight Actilane 421 (acrylate monomer, 300 parts by weight manufactured by Akcros) (E) Disper BYK-168 400 parts by weight

The components above were mixed by stirring to give a pigment mill base. Preparation of the pigment mill base was carried out by putting the pigment ink into an M50 disperser motor mill (manufactured by Eiger) and using zirconia beads having a diameter of 0.65 mm at a peripheral speed of 9 m/s for 10 hours.

Preparation of Yellow Mill Base C

(D) NOVOPERM YELLOW H2G 300 parts by weight Actilane 421 (acrylate monomer, 300 parts by weight manufactured by Akcros) (E) Disper BYK-168 400 parts by weight

The components above were mixed by stirring to give a pigment mill base. Preparation of the pigment mill base was carried out by putting the pigment ink into an M50 disperser motor mill (manufactured by Eiger) and using zirconia beads having a diameter of 0.65 mm at a peripheral speed of 9 m/s for 10 hours.

Preparation of Black Mill Base D

(D) SPECIAL BLACK 250 300 parts by weight Actilane 421 (acrylate monomer, 300 parts by weight manufactured by Akcros) (E) Disper BYK-168 400 parts by weight

The components above were mixed by stirring to give a pigment mill base. Preparation of the pigment mill base was carried out by putting the pigment ink into an M50 disperser motor mill (manufactured by Eiger) and using zirconia beads having a diameter of 0.65 mm at a peripheral speed of 9 m/s for 7 hours.

Preparation of White Mill Base E

(D) KRONOS 2300 500 parts by weight Actilane 421 (acrylate monomer, 400 parts by weight manufactured by Akcros) (E) Disper BYK-168 100 parts by weight

The components above were mixed by stirring to give a pigment mill base. Preparation of the pigment mill base was carried out by putting the pigment ink into an M50 disperser motor mill (manufactured by Eiger) and using zirconia beads having a diameter of 0.65 mm at a peripheral speed of 9 m/s for 4 hours.

Example 1

The components below were stirred using a high-speed water-cooled stirrer to give a cyan UV inkjet ink. The viscosity was 17 mPa·s.

Cyan Ink Composition

(D) (E) Cyan mill base A  6.0 parts FANCRYL 512A 35.4 parts (A) N-Vinylcaprolactam 25.0 parts NK ester AMP-10G 20.0 parts FIRSTCURE ST-1 0.05 parts (B) Lucirin TPO (photoinitiator manufactured by BASF)  8.5 parts (B) Benzophenone (photoinitiator)  3.0 parts (B) IRGACURE 184 (photoinitiator manufactured by CSC)  2.0 parts (F) BYK-307 (surfactant manufactured by BYK Chemie) 0.05 parts (C) Dimethylaminoethyl acrylate  0.1 parts Evaluation of Ink

Inkjet recording was carried out using the ink composition thus obtained. The results of evaluation of the decomposition of N-vinylcaprolactam and the curability are given in Table 1. The results of evaluation of image flexibility and adhesion before and after storage are also given in Table 1.

Example 2

The components below were stirred using a high-speed water-cooled stirrer to give a magenta UV inkjet ink. The viscosity was 19 mPa·s.

Magenta Ink Composition

(D) (E) Magenta mill base B 12.0 parts FANCRYL 512A 35.4 parts (A) N-Vinylcaprolactam 25.0 parts NK ester AMP-10G 14.0 parts FIRSTCURE ST-1 0.05 parts (B) Lucirin TPO (photoinitiator manufactured by BASF)  8.5 parts (B) Benzophenone (photoinitiator)  3.0 parts (B) IRGACURE 184 (photoinitiator manufactured by CSC)  2.0 parts (F) BYK-307 (surfactant manufactured by BYK Chemie) 0.05 parts (C) Dimethylaminoethyl acrylate  1.0 part Evaluation of Ink

Inkjet recording was carried out using the ink composition thus obtained. The results of evaluation of the decomposition of N-vinylcaprolactam and the curability are given in Table 1. The results of evaluation of image flexibility and adhesion before and after storage are also given in Table 1.

Example 3

The components below were stirred using a high-speed water-cooled stirrer to give a yellow UV inkjet ink. The viscosity was 19 mPa·s.

Yellow Ink Composition

(D) (E) Yellow mill base C 12.0 parts FANCRYL 512A 35.4 parts (A) N-Vinylcaprolactam 25.0 parts NK ester AMP-10G 14.0 parts FIRSTCURE ST-1 0.05 parts (B) Lucirin TPO (photoinitiator manufactured by BASF)  8.5 parts (B) Benzophenone (photoinitiator)  3.0 parts (B) IRGACURE 184 (photoinitiator manufactured by CSC)  2.0 parts (F) BYK-307 (surfactant manufactured by BYK Chemie) 0.05 parts (C) Dimethylaminoethyl acrylate  1.0 part Evaluation of Ink

Inkjet recording was carried out using the ink composition thus obtained. The results of evaluation of the decomposition of N-vinylcaprolactam and the curability are given in Table 1. The results of evaluation of image flexibility and adhesion before and after storage are also given in Table 1.

Example 4

The components below were stirred using a high-speed water-cooled stirrer to give a black UV inkjet ink. The viscosity was 17 mPa·s.

Black Ink Composition

(D) (E) Black mill base D  6.0 parts FANCRYL 512A 35.4 parts (A) N-Vinylcaprolactam 25.0 parts NK ester AMP-10G 20.0 parts FIRSTCURE ST-1 0.05 parts (B) Lucirin TPO (photoinitiator manufactured by BASF)  8.5 parts (B) Benzophenone (photoinitiator)  3.0 parts (B) IRGACURE 184 (photoinitiator manufactured by CSC)  2.0 parts (F) BYK-307 (surfactant manufactured by BYK Chemie) 0.05 parts (C) Dimethylaminoethyl acrylate  1.0 part Evaluation of Ink

Inkjet recording was carried out using the ink composition thus obtained. The results of evaluation of the decomposition of N-vinylcaprolactam and the curability are given in Table 1. The results of evaluation of image flexibility and adhesion before and after storage are also given in Table 1.

Example 5

The components below were stirred using a high-speed water-cooled stirrer to give a white UV inkjet ink. The viscosity was 24 mPa·s.

White Ink Composition

(D) (E) White mill base E 31.0 parts FANCRYL 512A 26.4 parts (A) N-Vinylcaprolactam 18.0 parts NK ester AMP-10G 10.0 parts FIRSTCURE ST-1 0.05 parts (B) Lucirin TPO (photoinitiator manufactured by BASF)  8.5 parts (B) Benzophenone (photoinitiator)  3.0 parts (B) IRGACURE 184 (photoinitiator manufactured by CSC)  2.0 parts (F) BYK-307 (surfactant manufactured by BYK Chemie) 0.05 parts FIRSTCURE ITX  1.0 part (C) Dimethylaminoethyl acrylate  1.0 part Evaluation of Ink

Inkjet recording was carried out using the ink composition thus obtained. The results of evaluation of the decomposition of N-vinylcaprolactam and the curability are given in Table 1. The results of evaluation of image flexibility and adhesion before and after storage are also given in Table 1.

Examples 6 to 26 and Comparative Example 1

An ink composition was prepared in the same manner as in Example 1 except that the type and amount of basic compound added were changed as shown in Table 1. In Example 23 to Example 26, FIRSTCURE ITX was not added.

Inkjet recording was carried out using the ink composition thus obtained. The results of evaluation of the decomposition of N-vinylcaprolactam and the curability are given in Table 1. The results of evaluation of image flexibility and adhesion before and after storage are also given in Table 1.

Comparative Example 2

An ink composition was prepared in the same manner as in Example 7 except that the amount of N-vinylcaprolactam was 10.0 parts by weight and the amount of NK ester AMP-10G was 18.0 parts by weight.

Inkjet recording was carried out using the ink composition thus obtained. The results of evaluation of the decomposition of N-vinylcaprolactam and the curability are given in Table 1. The results of evaluation of image flexibility and adhesion before and after storage are also given in Table 1.

Comparative Example 3

An ink composition was prepared in the same manner as in Example 7 except that the amount of N-vinylcaprolactam was 14.0 parts by weight and the amount of NK ester AMP-10G was 14.0 parts by weight.

Inkjet recording was carried out using the ink composition thus obtained. The results of evaluation of the decomposition of N-vinylcaprolactam and the curability are given in Table 1. The results of evaluation of image flexibility and adhesion before and after storage are also given in Table 1.

Evaluation Conditions

The ink compositions of the Examples and Comparative Examples were maintained at 60° C. for 4 weeks.

The curability, the proportion of N-vinylcaprolactam decomposed, the flexibility, and the adhesion were evaluated using these ink compositions.

The proportion of N-vinylcaprolactam decomposed in the ink composition was measured using liquid chromatography, and the change from the peak area of N-vinylcaprolactam in the initial state was ascertained.

Inkjet Image Recording Method

Recording was carried out on a recording medium using an experimental inkjet recording device having a piezo system inkjet nozzle with each ink composition before and after aging. The ink supply system comprised a main tank, a supply pipe, an ink supply tank immediately before an inkjet head, a filter, and a piezo system inkjet head, and a section from the ink supply tank to the inkjet head was thermally insulated and heated. Temperature sensors were provided on the ink supply tank and in the vicinity of the nozzle of the inkjet head, and the temperature was controlled so that the nozzle section was always at 45° C.±2° C. The piezo system inkjet head was driven so as to discharge multisize dots of 8 to 30 pL at a resolution of 720×720 dpi. The exposure system, the main scanning speed, and the discharge frequency were adjusted so that, after landing, UV light was focused to give an exposure area illumination intensity of 1,630 mW/cm², and irradiation started 0.1 sec. after the ink landed on the recording medium. The cumulative amount of light applied to an image was adjusted so as to be 4,500 mJ/cm². The UV lamp employed an HAN250NL high-cure mercury lamp (manufactured by GS Yuasa Corporation). Here, dpi referred to in the present invention denotes the number of dots per 2.54 cm. The recording medium employed an E5000 ester film (film thickness 125 μm, manufactured by Toyobo Co., Ltd.).

Method for Measuring Curing Sensitivity (Curability)

In accordance with the above-mentioned inkjet recording method, a solid printed image having an average film thickness of 12 μm was formed, and the stickiness of the image was evaluated by touch after the image was irradiated with ultraviolet rays and expressed using the following criteria with respect to the change between before and after aging.

3: No change between before and after aging.

2: Slight change between before and after aging, image was slightly sticky after aging.

1: Large change between before and after aging, and after aging the image was not hardened, to the extent that uncured ink was transferred to the hand.

Liquid Chromatography Measurement Method

The ink composition before and after aging was measured using a liquid chromatograph manufactured by Waters and a liquid chromatograph column RP-18 manufactured by Kanto Chemical Co., Inc. The peak areas of the N-vinylcaprolactam peaks before and after aging were compared, and the decrease (proportion decomposed) was calculated. The smaller the decrease, the more suppressed the decomposition.

Flexibility Evaluation Method: Bending Test

In the examples, as a method for evaluating the flexibility of a cured film before and after storage, a bending test was carried out.

In accordance with the above-mentioned inkjet image recording method, an E5000 ester film (film thickness 125 μm, manufactured by Toyobo Co., Ltd.) was used as a recording medium, and three solid printed images having average image area film thicknesses of 12 μm, 24 μm, and 36 μm were formed. The bending test involved bending once at 25° C. the recording medium on which an image had been formed, and an evaluation was carried out of the presence or absence of cracks in the image area. In general, when the average film thickness is large, the strain occurring in the image area when bending the image area becomes large, and cracks easily occur. That is, testing whether or not cracks occur in an image area having a larger film thickness gives a measure of the flexibility.

The evaluation criteria were as follows.

4: No cracks occurred for samples having an average film thickness of 12 μm, 24 μm, or 36 μm.

3: No cracks occurred for samples having an average film thickness of 12 μm or 24 μm.

2: No cracks occurred for a sample having an average film thickness of 12 μm, but cracks occurred in the bent portion of an image area of a sample having an average film thickness of 24 μm.

1: Cracks occurred in the bent portion of an image area for all the samples having an average film thickness of 12 μm, 24 μm, and 36 μm.

Substrate Adhesion Evaluation Method: Crosshatch Test (EN IS02409)

As a recording medium, PET (ester film E5000, film thickness 125 μm, manufactured by Toyobo Co., Ltd.) was used, and in accordance with the above-mentioned inkjet recording method, solid printed images having an average film thickness of 12 μm were formed on the substrate. Following this, each printed material was subjected to a crosshatch test (EN IS02409).

Viscosity Measurement Method

Measurement of viscosity in the examples was carried out using a Brookfield LVDV-I type B viscometer (manufactured by Brookfield) at 25° C. with a rotor rotational speed of 20 rpm.

Example 27

The components below were stirred using a high-speed water-cooled stirrer to give a cyan UV inkjet ink. The viscosity was 17 mPa·s.

Cyan Ink Composition

(D) (E) Cyan mill base A 6.0 parts FANCRYL 512A 35.4 parts (A) N-Vinylcaprolactam 25.0 parts NK ester AMP-10G 19.0 parts FIRSTCURE ST-1 0.05 parts (B) Lucirin TPO (photoinitiator manufactured by BASF) 8.5 parts (B) Benzophenone (photoinitiator) 3.0 parts (B) IRGACURE 184 (photoinitiator manufactured by CSC) 2.0 parts (F) BYK-307 (surfactant manufactured by BYK Chemie) 0.05 parts (C) Dimethylaminoethyl acrylate 1.0 part FIRSTCURE ITX 1.0 part Evaluation of Ink

Inkjet recording was carried out in the same manner as in the above-mentioned inkjet recording except that, after landing, UV light was focused to give an exposure area illumination intensity of 350 mW/cm² and a cumulative light intensity on an image of 1,500 mJ/cm², and the lamp employed an NCCU033 UV-LED lamp (manufactured by Nichia Corporation). The results of evaluation of the decomposition of N-vinylcaprolactam and the curability are given in Table 1. The results of evaluation of image flexibility and adhesion before and after storage are also given in Table 1.

Example 28

The components below were stirred using a high-speed water-cooled stirrer to give a cyan UV inkjet ink. The viscosity was 17 mPa·s.

Cyan Ink Composition

(D) (E) Cyan mill base A 6.0 parts FANCRYL 512A 35.4 parts (A) N-Vinylcaprolactam 25.0 parts NK ester AMP-10G 18.0 parts FIRSTCURE ST-1 0.05 parts (B) Lucirin TPO (photoinitiator manufactured by BASF) 8.5 parts (B) Benzophenone (photoinitiator) 3.0 parts (B) IRGACURE 184 (photoinitiator manufactured by CSC) 2.0 parts (F) BYK-307 (surfactant manufactured by BYK Chemie) 0.05 parts (C) Dimethylaminoethyl acrylate 1.0 part FIRSTCURE ITX 3.0 parts Evaluation of Ink

Inkjet recording was carried out in the same manner as in the above-mentioned inkjet recording except that, after landing, UV light was focused to give an exposure area illumination intensity of 450 mW/cm² and a cumulative light intensity on an image of 8,000 mJ/cm², and the lamp employed an SHP270W super high pressure mercury lamp (manufactured by PHOENIX Electric Co., Ltd.), which is generally used for a projector, etc. There were no dots missing in the image thus obtained, and a vivid image having a film thickness of 12 μm was formed. The results of evaluation of the decomposition of N-vinylcaprolactam and the curability are given in Table 1. The results of evaluation of image flexibility and adhesion before and after storage are also given in Table 1.

TABLE 1 Amount Amount of Proportion Flexibility Cross-hatch test Viscosity Basic Added NVC added Cura- decomposed Before After Before After Lamp (mPa · sec) Color compound (pbw) (pbw) bility (%) storage storage storage storage Ex. 1 HAN 250NL 17 Cyan A 0.1 25.0 2 5 4 4 5B 5B Ex. 2 HAN 250NL 19 Magenta A 1 25.0 3 0 4 4 5B 5B Ex. 3 HAN 250NL 19 Yellow A 1 25.0 3 0 4 4 5B 5B Ex. 4 HAN 250NL 17 Black A 1 25.0 3 0 4 4 5B 5B Ex. 5 HAN 250NL 24 White A 1 18.0 3 0 4 4 5B 5B Ex. 6 HAN 250NL 17 Cyan A 0.5 25.0 3 0 4 4 5B 5B Ex. 7 HAN 250NL 17 Cyan A 1 25.0 3 0 4 4 5B 5B Ex. 8 HAN 250NL 18 Cyan A 5 25.0 3 0 4 4 5B 5B Ex. 9 HAN 250NL 19 Cyan A 10 25.0 3 0 4 4 5B 5B Ex. 10 HAN 250NL 17 Cyan B 0.1 25.0 2 5 4 4 5B 5B Ex. 11 HAN 250NL 17 Cyan B 0.5 25.0 3 0 4 4 5B 5B Ex. 12 HAN 250NL 17 Cyan B 1 25.0 3 0 4 4 5B 5B Ex. 13 HAN 250NL 18 Cyan B 5 25.0 3 0 4 4 5B 5B Ex. 14 HAN 250NL 19 Cyan B 10 25.0 3 0 4 4 5B 5B Ex. 15 HAN 250NL 17 Cyan C 0.5 25.0 3 0 4 4 5B 5B Ex. 16 HAN 250NL 17 Cyan C 1 25.0 3 0 4 4 5B 5B Ex. 17 HAN 250NL 18 Cyan C 5 25.0 3 0 4 4 5B 5B Ex. 18 HAN 250NL 17 Cyan D 0.1 25.0 2 5 4 4 5B 5B Ex. 19 HAN 250NL 17 Cyan D 0.5 25.0 3 0 4 4 5B 5B Ex. 20 HAN 250NL 17 Cyan D 1 25.0 3 0 4 4 5B 5B Ex. 21 HAN 250NL 17 Cyan D 5 25.0 3 0 4 4 5B 5B Ex. 22 HAN 250NL 18 Cyan D 10 25.0 2 0 4 4 5B 5B Ex. 23 HAN 250NL 17 Cyan E 0.05 25.0 3 0 4 4 5B 5B Ex. 24 HAN 250NL 17 Cyan E 0.1 25.0 3 0 4 4 5B 5B Ex. 25 HAN 250NL 17 Cyan E 0.5 25.0 3 0 4 4 5B 5B Ex. 26 HAN 250NL 18 Cyan E 1 25.0 3 0 4 4 5B 5B Ex. 27 NCCU 033 17 Cyan A 1 25.0 3 0 4 4 5B 5B Ex. 28 SHP 270W 17 Cyan A 1 25.0 3 0 4 4 5B 5B Comp. Ex. 1 HAN 250NL 17 Cyan A — 25.0 1 30 4 2 5B 3B Comp. Ex. 2 HAN 250NL 19 Cyan A 1 10.0 1 0 1 1 3B 2B Comp. Ex. 3 HAN 250NL 18 Cyan A 1 14.0 2 0 2 1 3B 2B In Table 1, the basic compounds are as follows. Basic compound A: dimethylaminoethyl acrylate (DMA: Osaka Organic Chemical Industry Ltd.) Basic compound B: aminoacrylate (EBECRYL P115: Daicel-UCB Co., Ltd.) Basic compound C: aminoacrylate (EBECRYL 7100: Daicel-UCB Co., Ltd.) Basic compound D: triethanolamine Basic compound E: hindered amine-based compound (TINUVIN 770DF: Ciba Specialty Chemicals)

It is confirmed, as is clear from Table 1; that the ink compositions of Examples 1 to 26, which contain N-vinylcaprolactam as the N-vinyllactam, a polymerization initiator, and a basic compound showed good curability even after long-term storage, and decomposition of N-vinylcaprolactam as the N-vinyllactam is suppressed. The same level of curability can be obtained when a cured film is formed using an LED light source. It is further confirmed that the same level of curability can be obtained when a cured film is formed using a super high pressure mercury lamp.

On the other hand, it has been found that the ink composition of Comparative Example 1, which does not contain a basic compound, showed poor curability after long-term storage, and the N-vinylcaprolactam decomposed due to long-term storage.

It has also been found that in Comparative Examples 2 and 3, for which the N-vinyllactam is contained at less than 15 wt %, the curability is not sufficient, the resulting cured film has a low level of flexibility, and adhesion to a substrate is poor.

Example 29 Preparation of Support

A melt was prepared using an aluminum alloy containing Si (0.06 wt %), Fe (0.30 wt %), Cu (0.025 wt %), Mn (0.001 wt %), Mg (0.001 wt %), Zn (0.001 wt %), and Ti (0.03 wt %), the remainder being Al and its inevitable impurities, and it was subjected to a melt treatment and filtration, and then formed into an ingot having a thickness of 500 mm and a width of 1,200 mm by a DC casting method. After the surface thereof was shaved off by an average thickness of 10 mm by means of a scalping machine, it was thermally maintained at 550° C. for about 5 hours, and when the temperature dropped to 400° C., it was made into a rolled sheet having a thickness of 2.7 mm by means of a hot rolling mill. It was further thermally treated at 500° C. by means of a continuous annealing machine, and then finished so as to have a thickness of 0.24 mm by means of cold rolling, thus giving an aluminum sheet of JIS 1050 material. The aluminum thus obtained had an average crystal minor axis of 50 μm and an average crystal major axis of 300 μm. After making the width of this aluminum 1,030 mm, it was subjected to the surface treatment below to give an aluminum support.

Surface Treatment

The surface treatment involved consecutively carrying out the various treatments (i) to (x) below. After each treatment and washing with water, liquid was removed by a nip roll.

(i) Mechanical Roughening Treatment

The surface of the aluminum sheet was subjected to a mechanical roughening treatment by means of a rotating roll-shaped nylon brush while supplying a suspension of an abrasive (pumice) having a specific gravity of 1.12 in water as an abrasive slurry to the surface of the aluminum sheet. The abrasive had an average particle size of 30 μm and a maximum particle size of 100 μm. The material of the nylon brush was nylon-6,10, the bristle length was 45 mm, and the diameter of the bristles was 0.3 mm. The nylon brush was formed by making holes in a stainless steel tube having a diameter of 300 mm and densely implanting the bristles. Three rotating brushes were used. The distance of two support rolls (φ 200 mm) below the brush was 300 mm. The brush rolls were pressed against the aluminum sheet so that the load on a drive motor for rotating the brushes increased by 7 kW from the load before pressing the brush rolls. The direction of rotation of the brushes was the same as the direction in which the aluminum sheet moved. The rotational speed of the brushes was 200 rpm.

(ii) Alkali Etching Treatment

The aluminum sheet obtained above was subjected to an etching treatment by means of a spray using an aqueous solution having a sodium hydroxide concentration of 2.6 wt %, an aluminum ion concentration of 6.5 wt %, and a temperature of 70° C. so as to dissolve 10 g/m² of the aluminum sheet. Subsequently, it was washed with water by means of a spray.

(iii) Desmutting Treatment

A desmutting treatment was carried out by means of a spray using an aqueous solution having a nitric acid concentration of 1 wt % and a temperature of 30° C. (containing 0.5 wt % of aluminum ion), and following this washing with water was carried out by means of a spray. The aqueous solution of nitric acid used in the desmutting treatment employed liquid waste from a step involving carrying out an electrochemical roughening treatment using alternating current in an aqueous solution of nitric acid.

(iv) Electrochemical Roughening Treatment

An electrochemical roughening treatment was carried out consecutively using an ac voltage of 60 Hz. An electrolytic solution in this process was a 10.5 g/L aqueous solution of nitric acid (containing 5 g/L of aluminum ion and 0.007 wt % of ammonium ion), and the solution temperature was 50° C. The electrochemical roughening treatment was carried out using as an ac power source waveform a trapezoidal rectangular wave alternating current having a duty ratio of 1:1 and a time from zero to peak current value of 0.8 msec, with a carbon electrode as a counter electrode. Ferrite was used as an auxiliary anode.

The current density was 30 A/dm² as a peak current value, and the quantity of electricity was 220 C/dm² as the total quantity of electricity when the aluminum sheet was the anode. 5% of the current flowing from the power source was diverted to the auxiliary anode. Following this, washing with water was carried out by means of a spray.

(v) Alkali Etching Treatment

The aluminum sheet was subjected to an etching treatment at 32° C. by means of a spray using an aqueous solution having a sodium hydroxide concentration of 26 wt % and an aluminum ion concentration of 6.5 wt % so as to dissolve 0.50 g/m² of the aluminum sheet, remove a smut component containing aluminum hydroxide as a main component formed in the previous paragraph when carrying out the electrochemical roughening treatment using alternating current, and dissolve an edge portion of a pit formed to thus make the edge portion smooth. Subsequently, washing with water was carried out by means of a spray.

(vi) Desmutting Treatment

A desmutting treatment was carried out by means of a spray using an aqueous solution having a nitric acid concentration of 15 wt % and a temperature of 30° C. (containing 4.5 wt % of aluminum ion), and following this washing with water was carried out by means of a spray. The aqueous solution of nitric acid used in the desmutting treatment employed liquid waste from the step involving carrying out the electrochemical roughening treatment using alternating current in an aqueous solution of nitric acid.

(vii) Electrochemical Roughening Treatment

An electrochemical roughening treatment was carried out consecutively using an ac voltage of 60 Hz. The electrolytic solution in this process was a 5.0 g/L aqueous solution of hydrochloric acid (containing 5 g/L of aluminum ion), and the temperature was 35° C. The electrochemical roughening treatment was carried out using as an ac power source waveform a trapezoidal rectangular wave alternating current having a duty ratio of 1:1 and a time from zero to peak current value of 0.8 msec, with a carbon electrode as a counter electrode. Ferrite was used as an auxiliary anode.

The current density was 25 A/dm² as a peak current value, and the quantity of electricity was 50 C/dm² as the total quantity of electricity when the aluminum sheet was the anode. Following this, washing with water was carried out by means of a spray.

(viii) Alkali Etching Treatment

The aluminum sheet was subjected to an etching treatment at 32° C. by means of a spray using an aqueous solution having a sodium hydroxide concentration of 26 wt % and an aluminum ion concentration of 6.5 wt % so as to dissolve 0.12 g/m² of the aluminum sheet, remove a smut component containing aluminum hydroxide as a main component formed in the previous paragraph when carrying out the electrochemical roughening treatment using alternating current, and dissolve an edge portion of a pit formed to thus make the edge portion smooth. Subsequently, washing with water was carried out by means of a spray.

(ix) Desmutting Treatment

A desmutting treatment was carried out by means of a spray using an aqueous solution having a sulfuric acid concentration of 25 wt % and a temperature of 60° C. (containing 0.5 wt % of aluminum ion), and following this washing with water was carried out by means of a spray.

(x) Anodizing Treatment

An anodizing treatment was carried out using an anodizing system (first and second electrolysis section lengths 6 m each, first and second power supply section lengths 3 m each, and first and second electrode section lengths 2.4 m each). Sulfuric acid was used as an electrolytic solution supplied to the first and second electrolysis sections. Each of the electrolytic solutions had a sulfuric acid concentration of 50 g/L (containing 0.5 wt % of aluminum ion) and a temperature of 20° C. Following this, washing with water was carried out by means of a spray. The final amount of oxidized film was 2.7 g/m².

Hydrophilized Layer by Means of Silicate

The support thus treated was immersed in an aqueous solution of No. 3 sodium silicate at 70° C. for 13 sec., then washed with water and dried. The surface roughness Ra obtained as an average value of 5 measurements using a Surfcom model 575A surface roughness meter manufactured by Tokyo Seimitsu Co., Ltd. with a cutoff value of 0.8 mm for a measurement length of 3 mm was 0.55 μm.

Ink-Receiving Layer

An ink-receiving layer coating solution having the composition below was applied using a wire-wound bar and dried at 80° C. for 15 sec. to thus form a coated film, thereby giving a support with an ink-receiving layer. The amount of ink-receiving layer applied was 20 mg/m².

TABLE 2 Amount Classification Name added Water-soluble Poly(sodium p-styrenesulfonate) 0.25 g polymer Surfactant Compound [F-1] 0.2 g Coloring material Acid Violet 34 Dye 0.05 g Coating activator TSA-731 0.0005 g (silicone-based surfactant, manufactured by Toshiba Silicone Co., Ltd.) Medium Ion-exchanged water 60 g Medium Methanol 40 g

Inkjet Recording

Inkjet recording was carried out, on a support on which the above-mentioned ink-receiving layer had been formed, using the same ink composition as that of Example 1 as follows.

When forming an image, as a head a shear mode piezo head (CA3: minimum droplet size 6 pL, number of nozzles 318, nozzle density 150 nozzles/25.4 mm, manufactured by Toshiba Tec Corporation) was used, and a head scanning type image formation system equipped with this head on a mobile carriage was employed. The ink was charged into an ink tank with a capacity of 2 L that had a pressure reduction function and the ink, which was degassed by removing gas that had dissolved in the ink by reducing the pressure to −40 kPa, was introduced into the above-mentioned head by a flexible tube of tetrafluoro ethylene having an inner diameter of 2 mm via a hydrostatic pressure control tank (capacity 50 mL). By controlling the height of the hydrostatic pressure tank relative to the head, the internal pressure of the head was adjusted to −6.6 kPa, and the meniscus shape in the nozzle portion of the head was controlled. Furthermore, water was circulated in the head by a circulating water-type temperature control system (SCINICS CH-201) so that the ink temperature within the head became 45° C. The drive voltage for the head was 24 V, and discharge was carried out in 8-value multi drop mode or binary mode. The frequencies for dot formation were 4.8 kHz and 12 kHz respectively. The imaging pitch was 600 dpi in the head scan direction (head scan speed 203 mm/s)×600 dpi in the recording medium transport direction for the 8-value multi drop mode, and 1,200 dpi in the head scan direction (head scan speed 254 mm/s)×1,200 dpi in the recording medium transport direction for the binary mode, that is, bidirectional interlace printing was carried out by the head while stepping the recording medium. Furthermore, as cleaning means for the above-mentioned head, wiping means comprising a nonwoven cloth that carried out wiping without contacting the nozzle plate of the head was provided, and cleaning was carried out as appropriate.

Exposure

1 sec. to 60 sec. after the inkjet recording, exposure to light was carried out using a 3 kW high pressure mercury lamp to thus form a lithographic printing plate.

Image Evaluation

The diameter of image dots thus obtained was measured using an optical microscope, and was found to be 35 μm.

Printing Test

Printing was carried out using the lithographic printing plate thus obtained, without subjecting it to a gum treatment, with a Lithron printer manufactured by Komori Corporation, using IF102 dampening water (manufactured by Fuji Photo Film Co., Ltd.), and DIC-GEOS (N) black ink manufactured by Dainippon Ink and Chemicals, Incorporated. 10,000 sheets or greater of high quality printed material free from white dropouts in an image area and stains in a non-image area were obtained, and it has been ascertained that the plate life is at a level that causes no problems in practical use. 

1. An ink composition for inkjet recording comprising: (A) an N-vinyllactam; (B) a polymerization initiator; (C) a basic compound; and (E) a basic dispersant; wherein the content of the N-vinyllactam (A) is at least 15 wt % but no greater than 40% of the total weight of the ink composition, the viscosity of the ink composition at 25° C. is 3 to 50 mPa·s, the basic compound is an organic amine compound having a polymerizable group is at least one selected from the group consisting of N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, and an aminoacrylate in which a polyfunctional acrylate monomer or polyester acrylate is modified with an amine, the basic dispersant is a polymeric dispersant having an amine value that is larger than its acid value, the difference between the acid value and the amine value of the polymeric dispersant is at least 5 mg KOH/g, and the polymerization initiator (B) consists of a radical polymerization initiator.
 2. The ink composition according to claim 1, further comprising (D) a colorant and (F) a surfactant.
 3. The ink composition according to claim 1, wherein the N-vinyllactam is N-vinylcaprolactam.
 4. The ink composition according to claim 1, wherein the viscosity of the ink composition at 25° C. is 7 to 30 mPa·s.
 5. An inkjet recording method comprising: (a¹) step of discharging the ink composition according to claim 1 onto a recording, medium; and (b¹) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation.
 6. The inkjet recording method according to claim 5, wherein the actinic radiation is UV radiation having a peak light emission wavelength in the range of 350 to 420 nm and is emitted by a UV radiation-emitting light-emitting diode that gives a maximum illumination intensity on the surface of a recording medium of 10 to 2,000 mW/cm².
 7. A process for producing a lithographic printing plate, the process comprising: (a²) step of discharging the ink composition according to claim 1 onto a hydrophilic support; and (b²) a step of curing the ink composition by irradiating the discharged ink composition with actinic radiation so as to form a hydrophobic image on the hydrophilic support by curing the ink composition.
 8. The ink composition according to claim 1, wherein the radical polymerization initiator is an aromatic ketone.
 9. The ink composition according to claim 1, wherein the ink composition comprises at least three types of radical polymerization initiators.
 10. The ink composition according to claim 1, wherein the radical polymerization initiator includes at least one type of an acylphosphine compound and at least two types of aromatic ketones. 